Depletion Gilding Notes

This is an academic paper, not a technical 'how to' guide. Many of the procedures described are unsafe in practice.

A Warning

The procedures and recipes in this paper were gathered from a number of historical and technical sources many of which were written before safety became an issue in the workshop. They are compiled for academic interest and are not intended as a listing of technical procedures for goldsmiths to follow. Any possible application would require the use of an excellent chemical fume hood and suitable safety and environmental precautions. An understanding of chemistry and a professional attitude would be absolutely necessary before investigating any procedure described in this paper. The purpose of this paper is to foster an understanding of the process of depletion gilding through contrast and comparison using historical references.

Introduction

All goldsmiths throughout the world seem to have some method of depletion gilding surfaces, that is of treating the surface of a finished gold object to remove the non-gold alloy metals leaving an ever richer layer of gold at the surface which may be compacted by burnishing or light polishing to give the impression the metal is a higher carat gold than it is. This adds to the aesthetic and monetary value of the object and improves the resistance of the alloy surface to corrosion caused by acids and chemicals. Unless the enrichment is carried out to unusual lengths it may be removed by abrasion. If enrichment is carried out to the extent that all the metals other than gold are removed from the alloy then the process is termed refining.

Western Goldsmiths refer to depletion gilding as 'coloring the gold'. The process is closely related to refining procedures and must have been one of the earliest methods of changing the surface appearance of gold alloys. Depletion gilding usually hinges on the conversion of the base metals near the surface of the gold alloy to chlorides (salts) or oxides using chemical attack and/or heat. These relatively vulnerable compounds are then removed from the surface leaving a layer of pure gold behind.

While all goldsmiths use some version of this the Japanese and the South American Indians developed alloys exploited specifically for surface enrichment. The South American Indians (or at least their western historical recorders) differentiated between two main types of gold alloy with which they worked. Guanin had a high silver content and Tumbaga had a high copper content. It may be of interest to note that the Indonesian word for copper is 'Tambaga' and that the word 'Tumbaga' may be an Indonesian word transported by the Spanish to South America to describe a reddish copper-gold alloy found there (Personal observation 1979).

These metals were widespread and the proportion of them varied according to period, for example fully half of these alloys were gold rich in earlier cultures (300BC-400AD, 555-90% gold) and copper or alloy rich in later cultures (400AD on, 45%-90% copper: 55%-10% gold) (Root, pp 252-256). It is of some interest to note that recent articles propose that the use of tumbaga had spiritual and symbolic imporatnce as the gold was part of the metal throughout rather than merely on the surface and that the smell of tumbaga had female and sexual connotations to the indians (Bray/Jones, p 84).

Two main depletion gilding techniques were used, one based on the conversion of the base metal alloy additions to salts which are absorbed into clay particles next to the metal. This process is termed cementation. The other approach used vegetable acids and involved the oxidation of the base metals and their leaching over a long period of time, weeks if not longer. The former process is most suitable for silver rich guanin and gold rich alloys and the latter works well for copper rich tumbaga alloys though it will work on most alloys given time and repetition.

The organic acids used included urine (Emmerich, p 165), acetic acid or vinegar (Tushingham, P 56) and oxalic acid from chewed plants of the oxalis family (Root, p 253, Ganzenmüller, p 42). The cementation process as used in South America involved the mixing or covering of impure gold (for refining) or gold objects in order to depletion gild them with salt and clay combined with heating which eventually enriches the gold surface. For refining the procedure was repeated until the gold remaining was almost pure (Plazas, Falchetti, p 46). On the Peruvian coast the same process was used for depletion gilding and of note is that at least there the clay used for this purpose contained a high proportion of potassium nitrate and sulfates of potassium, iron and copper (Tushingham, p 59). As will be seen later in this paper this constitutes an almost perfect depletion gilding mixture.

What is of interest to me in examining depletion gilding is that so many experiments were carried out in every area of the world to come up with procedures that share the same core processes. As a practicing goldsmith concerned with technique the examination of Process is vital to developing free technical choice.

There is a fundamental difference between Process and procedure. Process is what really goes on, what actually happens when one affects the metal. Process can be described in scientific terms or even paraphrased to create an easily understood mental model of what is occurring which then allows the user to control and guide the process and procedures used. A procedure is a way of effecting a process, a recipe, a technique.

There may be dozens of procedures to obtain a similar end effect but there will be only one process or series of processes occurring. It should be noted that whether the language used to describe the process is accurate scientifically or is phrased in terms of magic and supposed superstition the same control potential is there. The mind needs concepts to grasp and manipulate, almost as if they were physical objects in order to develop control of process. What those terms are does not really matter providing they describe some accurate model of what actually occurs.

If one thinks and knows only recipes and procedures then one can be stopped by a technical problem. If on the other hand one goes to the Process one can then solve most technical problems relatively easily.

A story to illustrate this that of a person taught to fry an egg before leaving home for the first time and being inexplicably unable to cook. The first week or so is fine but then fried eggs start to pall. The only way the person will learn how to cook an egg another way without being actively taught is to go to the process; to say that 'fried egg' is heat plus egg. Then one can list all possible variations of this to obtain options for procedures to cook the egg. These might include piling up gunpowder over the egg and setting fire to it, rubbing it between boards to cook it by the heat of friction, the use of magnifying glasses or even the proverbial sidewalk.

There would amongst the solutions however also be poaching, roasting, baking, bonding to other foods and cooking etc. By going to the process one discovers unlimited procedural options. Some will not be appropriate for one reason or another (gunpowder eggs cooked like that might leave something to be desired) and it is one's job to make the appropriate decision on technical choice.

This difference between process and procedure is something that one has to actively learn to think about after undergoing a traditional goldsmiths or art school education which is often tremendously procedure bound. As we know there is no 'right' way to do things, merely variations of more or less appropriateness to the technical problem in hand. By attempting to always determine the process new and different solutions to problems can be found and at some point or another some or all of the apparently ridiculous answers may prove to be the correct solution under a different set of conditions.

I believe that goldsmithing skills should all be taught by the use of process rather than procedure. An example is when I have an introductory class and we discuss the jewellers saw frame. We begin by discussing process and procedure and then we talk of 'separating sheet and solid material'. We list every possible way of doing this from lasers to gnawing through with ones teeth (not efficient). Then we discuss the options available and the nature of the sawblade which is a chip forming tool like a chisel or a file. In fact a sawblade and a file are the identical tool it is only that a blade is a thin slice of a file. In this way an understanding of process is built up and whenever possible all techniques are discussed in the light of this.

I think that in this way people are not trapped by technique or problems with lack of skill. It seems that even people with limited skill levels when taught this way are able to begin to solve their technical problems well and far more rapidly than in my previous experience.

In the case of gold depletion gilding I have found it useful to examine an eclectic but by no means exhaustive collection of procedures. By looking at the procedures and reading some basic chemistry material an understanding of the Process of depletion gilding is achieved. The purpose of depletion gilding can be stated quite simply: goldsmiths find it necessary to remove alloy additions from gold alloys in order to obtain pure gold with which to work or to change the color of alloy surfaces for aesthetic or economic reasons. This removal of alloy additions may be from the entire chunk of metal in which case we call it 'refining' or merely from the surface when we term it 'coloring ' the gold or 'depletion gilding'.

The need for refining is obvious-to produce a pure enough gold to allow control over further alloying procedures for specific ends. The need for depletion gilding is essentially to make a lower carat gold appear to be a higher carat; to make gold alloys appear to be pure gold. This may be for aesthetic reasons (or cultural as in the case of South America) or even for fraudulent ones. It is also possible to selectively depletion gild gold surfaces for design choice.

The process itself relies upon gold's resistance to oxidation and attack by acids. The alloy addition metals are oxidized or converted to salts by chemical (acids) and heat action, in the case of refining often aided by melting. The chlorides are in general then removed by fluxes and come off as slag or are absorbed by porous clay particles in contact with the melt.

The other main approach is to oxidize the copper particles in the alloy and remove them using appropriate acids, usually sulfuric or organic in nature like the juice of the bitter plum used in Japan,plants used in SouthAmerica or the western equivalent: rhubarb. These latter organic acids contain among other things malic and oxalic acids. These acids may be used in their pure and diluted forms to remove both the copper and the silver from the alloy or in the case of mineral acids may be released by mixtures of chemicals which break down upon heating onto the surface of the object to be depletion gilded.

Other techniques include using sulfur to remove silver from the alloy in a technique related to making niello and stripping the more easily detached non-gold atoms from the surface electrolytically in a bath which contains acids, bases or salts. Some mixtures dissolve some gold along with the copper and silver and when the dissolved gold content reaches a specific point it plates out onto the gold alloy surface due to electrochemical replacement reactions. This process is in addition to the enrichment that occurs by depletion of the addition metals in the alloy.

The main alloy additions to gold are copper and silver. Copper is added to make a red colored gold and silver to make a green. There is of course a complete range of mixtures of this three part alloy which offer a range of colors from greenish to yellows to reds depending upon the specific mix of metals. Depletion gilding offers the opportunity to enrich the gold content at the surface of the object or to preferentially attack one of the alloy constituents thus tilting the surface color to the tones given by the remaining alloy addition. For example, if silver is removed from an alloy a tendency towards reddish tones is produced as the remaining copper affects the color of the gold alloy.

The main processes for depletion gilding then are: cementation, cupellation (using lead as a fluxing agent to remove oxides), salt mixtures that release mineral acids, oxidation of the copper and its removal by acids, the use of mineral acids themselves, sulfur for removal of silver in the form of sulfides, and electrolytic stripping (chemical and with applied current).

The rest of this paper is composed of a number of discrete short sections each dealing with a specific depletion gilding procedure or recipe gathered from a number of different sources. Most useful was Gmelins Handbuch der Anorganische Chemie published in 1950. This enormous set of volumes describes what was known about each element, by element. There are two volumes on gold, one on history and one on chemistry. Both are carefully constructed and cover much of the world's scientific and historical sources to 1950.

Please view this document as a comment on Process and procedure and as a potentially interesting limited review of depletion gilding procedures from various cultures. Contrast and comparison are facillitated by the detailing of procedures which follows. The various procedures are listing by major types.

Cementation and Removal of Non-gold Metals from Gold Alloys

Cementation is properly a refining procedure but as any refining procedure is at heart also a depletion gilding procedure cementation is used in varying forms as a method of depletion gilding objects. The difference between refining and depletion gilding is usually only a matter of degree: in depletion gilding one is concerned with increasing the gold content at the surface to a depth that effectively addresses concerns with wear on the particular article while in refining one is concerned with converting the entire alloy to as pure a gold as one can obtain with the technique chosen. Alchemists experimented widely with methods of doing both and much research on depletion gilding (perhaps even occasionally used for fraudulent purposes) was carried out in the name of alchemy.

In cementation the silver and copper containing gold alloy is heated in the presence of a selection of active salts which become acids at high temperatures which form among other things silver chlorides (HCl for example may be produced by water and salt.). Copper and silver chlorides formed are absorbed by the crucible or brickdust accompanying the melt leaving the gold behind. The process needs oxygen to work well and thus needs a porous crucible. Without the presence of the porous material the reaction slows or stops. The presence of salt promotes the most effective procedure. Water helps the breakdown of the salt and may even be supplied to the mixture from the gas flame (Ganzenmüller p 60). The procedure works best for gold alloys which are more than 50% fine. The maximum gold fineness attainable is between 87.6% and 91.7% fine. Repeated treatments with silver-gold alloys can however leave almost pure gold behind (Ganzenmüller, pp345) It is however difficult to recover any silver from the silver chlorides absorbed by the clay present (Ganzenmüller p 59-60). Note that in some processes where chlorine is liberated the gold is also dissolved and then plates out over the metal surface from solution. This action may be in addition to the removal of silver and copper from the alloy in the form of chlorides. Chlorine is used today in refining crude mine bullion in the Miller process. The chlorine is injected into the molten alloy which forms chlorides first of any iron, lead and zinc present and then of the copper and silver. These chlorides separate out into the slag. This process as commercially used usually renders gold purities of between 99.50% and 99.80%. Higher concentrations (99.99%) are achieved by using electrolytic refining methods (Rapson, pp 28-29).

Cupellation

Cupellation is used as a method of assaying, that is obtaining the gold content of an unknown alloy. It is also a refining procedure in in it's own right and could be used in a modified version as a depletion gilding procedure. It is in fact the oldest refining procedure mentioned in the literature with short descriptions by Agatharchides and Pliny. In cupellation the alloy is wrapped in lead foil and heated in a porous ash and clay crucible in the presence of a glassy flux. The lead covering dissolves copper and other oxides and other constituent metals become oxides which are absorbed by the porous container leaving pure gold behind. The gold may contain some silver which must then be separated from the gold using acids (usually boiling nitric acid) (Ganzenmüller p 58).

Mansur Al Kamily, a chief chemist at the Cairo mint in the 13th century composed a handbook which describes the process of cupellation as used for refining. "One takes sieved ashes or chalk, or the burnt, powdered bones of animals, or these materials mixed together or some of them, moistens them with a little water and compresses it with the hands to form a solid, tight mass. One makes a rounded depression in the middle and strews some ground up glass on the floor of the indentation. Then one lets it dry.

When it is dry one places the metal to be tested in the described indentation. One makes a strong charcoal fire around it and makes an air blast over the metal to be tested until it melts. When it is molten one adds some lead little by little and blows upon it with a stronger flame. If one sees the molten metal moves about seethes and boils it is not yet clean. One waits until all the lead has come off as fumes. One then adds more lead and blasts the flame over it until the lead has separated itself from the melt. If it is not yet calm and quiet then again lead is added and heated until the molten metal is quiet and one can see that it's surface is clean and clear. Then one takes the charcoal fire apart and pours water over it. In this way the test is correctly carried out. If during the procedure one occasionally throws on some glass the metal is purified even more as the glass removes the slag and surrounds it. One can instead of glass add salt or borax or some alum".

The air blast effects an oxidizing melt so that lead oxide is created. The lead slag dissolves the copper and other metal oxides formed which are then absorbed by the porous crucible. Any chlorides formed are also absorbed into the clay present. Some silver may be retained by the gold, either from silver in the initial alloy or silver present in the lead (Ganzenmüller, p58).

Theophilus describes an almost identical procedure for refining silver which would work for gold alloys as above: "Sift some ashes, mix them with water and take a fire-tested earthenware dish of such a size that you believe the silver which is to be refined can be melted in it without running over. Put ashes in it, thinly in the middle and thickly around the rim, and dry it in front of the fire. When it is dry, remove the coals a little distance from the [wall of the] forge and put the dish with its ashes in front of the wall beneath the [tuyere] hole, so that the blast from the bellows will blow onto it. Then replace the coals on top, and blow until the dish is red-hot. Then put the silver into it, add a little lead on top, heap charcoal over it and melt it. You should have at hand a stick cut from a hedge and dried in the wind, with which you should carefully uncover the silver and clean away whatever impurity you see on it. Then put on it a brand, that is, a stick burnt in the fire, and blow gently, using a long stroke {of the bellows}.

When you have removed the lead by this blowing, if you see that the silver is not yet pure, again add lead, place the charcoal on it, and do as you did before. However, if you see the silver boiling and jumping out, know that tin or brass is mixed with it and finely crush a small piece of glass and throw it on the silver. Then add lead, put charcoal on, and blow vigorously. Then inspect it as before, take away the impurities of glass and lead with the stick, put the brand on it, and do as before" (Theophilus, Hawthorne and Smith, p 96). An almost identical procedure is described for separating gold from copper. In order to obtain the bone ash for the crucibles he recommends "take the bones of any kind of animal that you may have found in the street and burn them" which says much about public hygiene in the early twelth century. The major difference in approach to that described before is that the gold is wrapped in thinly beaten lead foil (Theophilus, Hawthorne and Smith, p 146).

Some Specific Cementation Procedures

Note that cementation as all refining procedures is a type of extended or extreme depletion gilding these recipes may be used in the form of applied pastes or immersion baths to selectively or fully depletion gild an object. I am referring to cementation here as a procedure in which metal salts (specifically chlorides) are formed which are then absorbed into porous clay present in or surrounding the melt in the form of a melting crucible. Salt containing recipes develop chlorine on the surfaces which dissolves some gold which then plates out on the surface again (Ganzenmüller p70). While cementation normally utilizes clay to absorb the silver chlorides in a similar process using Potassium Nitrate KNO₃ and Sodium Chloride NaCl the reaction is:

KNO₃ + 2HCL + NaCl => NOCl + Cl₂ + NaOH + KOH

This release of chlorine has the same effect as Aqua Regia does; all metals including the gold is converted into chlorides. The silver chloride that is formed is dissolved by the sodium chloride and so cannot interfere with the coloring process. If the solution at the surface contains enough H[AuCl₄] an ion exchange occurs between the copper ions of the metal object and the gold ions of the solution. The gold is then forced from the solution and plates out on the surface of the metal. This is another reason for high copper content alloys being better suited for certain kinds of depletion gilding than high silver content ones (Brepohl, p 315).

Granulate the gold-silver alloy, heat with salt; sodium chloride (NaCl) and clay. Silver Chloride AgCl is formed and absorbed by the clay. The process is repeated to fully refine the gold (Ganzenmüller p 345).

Mixtures of ammonium chloride, salt and clay are also used as are plain salt and clay. In both cases silver chloride is formed which is absorbed by the brick dust. It is noted that this procedure may not be very economical as only purities between 87.6% and 91.7% are easily achievable and it is very difficult to extract silver chlorides from the clay or brick dust. The addition of alum and ferrous sulfate is recommended. The salt breaks down forming hydrochloric acid HCl and the ferrous sulfate break down to form sulfuric acid H₂SO₄. The addition of ammonium chloride (and ferrous sulfate) ensures that enough HCl is produced to dissolve the silver (Ganzenmüller, pp 59-60).

The Alchemists investigated cementation techniques. Cementation chemicals included brickdust, Armenian Bolus (could not find a proper name for it), salt, ferrous sulfate, soot, Potassium Nitrate and Ammonium chloride. Sometimes a combination of brickdust and salt was used alone. An example might be:

1. 1 part Brickdust
2. 1/3 part Salt
3. l/8 part Ferrous Sulfate

Agricola has another:

• 1/2 pound brickdust
• 1/4 pound Salt
• 1 ounce Potassium Nitrate
• 1/2 ounce Ammonium chloride
• 1/2 ounce 'Steinsalz' (could not find proper name)

The above mixtures might also have ferrous sulfate, vinegar or urine added (Ganzenmüller p 59).

Other Salt Containing Depletion Gilding Examples.

A number of other salts are used in combination with sodium chloride. Potassium nitrate (saltpeter) KNO₃ is used as an oxidizing flux.. It is the potassium salt of nitric acid. One uses saltpeter for oxidizing melting (driving off base metals) because it melts at a low temperature of 339o and is then able to oxidize non-precious metals in that it changes itself to potassium nitrite thus losing an oxygen atom, which is then available for oxidation as the salt of the nitrous acid. Using lead as an example a typical reaction is:

KNO₃ +Pb=>KNO₂+PbO

[potassium nitrate]+[lead]=>[potassium nitrite]+[lead oxide]

Sodium nitrate NaNO₃ is also used in some recipes. As the sodium salt of nitric acid, sodium nitrate is very similar to potassium nitrate. It works like the related potassium nitrate and melts at 316oC. If 1 part sodium and 1 part potassium nitrate are melted together one even obtains a melting point reduction of the mixture to 218oC. (Brepohl, p 315).

Cover the alloy with:

• 8 parts KNO₃
• 7 parts Sodium Chloride,NaCl (table salt)
• 5 parts Alum

Heat the object until the applied material melts and then wash it off (Ganzenmüller p 403).

This solution is used boiling to dissolve the surface of alloys of 14 carat and over without roughening the surface. It is not suggested for 12 carat and lower as it would pit and damage the surface.

• 4 parts Potassium Nitrate KNO₃
• 2 parts Sodium Chloride,NaCl
• 1 part Hydrochloric Acid HCL

The dry materials are ground together and heated with a little boiling water. Then the acid is added and the mixture will boil up to the top of the container. The gold which has been previously cleaned in a lye solution is suspended in the mixture on a platinum or silver wire for a minute or so. This is repeated with water additions to the solution until the work is the correct color (Maryon, p 261).

Combine in a heated crucible:

• 2 parts Potassium Nitrate KNO₃ (Saltpeter)
• 1 part Sodium Chloride NaCl
• 1 part Alum

Heat the mixture until it is fluid. The work is dipped into a 1:10 nitric acid mixture, rinsed in boiling water, agitated in the crucible of molten salts for a few minutes. Then it is removed, rinsed and repeated as necessary to obtain the desired color (Von Neumann, p 58).

Maryon (who may be the source for Von Neumann) offers:

• 2 parts Potassium Nitrate
• 1 part Sodium Chloride
• 1 part Alum

These are ground together and heated in an appropriate container. The work is treated in the molten solution with alternating dips in nitric acid (Maryon, p 262).

This recipe comes from a Gudgerati goldsmith at a company called Lords I visited in Fiji in 1980.

• 1 part Sodium Chloride
• 1 part Alum
• 1 part Potassium Nitrate

Ammonium chloride was sometimes also used to melt with as was something that appeared to be copper sulfate.

Morton notes: "An ancient formula for turning gold to a raw yellow color consisted of boiling the gold in a salt solution with ammonia" (Morton, p279).

While not specifically depletion gilding Hickman reports the removal of copper from low silver content metal in Japan leaving fine silver as a thick layer on the surface of the metal. When certain military rulers needed funds silver copper alloy bars of metal were cast with silver contents as low as 15-20%. These were cast into molds set under hot water to prevent surface oxidation. The bars were then heated to red heat and quenched in plum vinegar and salt solution. After several hours in this they were boiled in plum vinegar without salt, washed and dried. It should be noted that this procedure was also followed with most coins and other silver objects even when of high purity (Hickman, pp84-85). A similar process would remove the metal additions such as copper and silver from a gold alloy.

The Leidensis Papyrus (3rd Century BC) mentions the identical plum vinegar method for removing silver from gold alloys (Ganzenmüller, p 41).

In the 19th Century in Japan poor alloys of gold appeared where the gold had been debased to 8 ct (33%) and even 12.3% or 3 ct gold. However because of surface enrichment they appeared to be pure gold (Hickman, p 76). Around 1980 when gold was becoming very expensive some German Refiners were rumored to be experimenting with 3 and 4 ct gold alloys. They gave up on them as commercial products as apparently even the acids and salts of the skin were enough to pit and corrode them. Depletion gilding such an alloy may then not have been as difficult as one might imagine.

Combine the following:

• 1 part Sulfur
• 1 part Potassium Bitartrate
• 2 parts Salt

This may be applied as a paste. This as all salt containing recipes is an example where chlorine is developed on the surfaces, dissolving the gold which then plates out on the surface as pure gold (Ganzenmüller p70).

When the gold content is no higher than 14K, (585/1000 parts gold) the surface may be depletion gilded with:

• 2 parts Sodium Chloride
• 4 parts Sodium Nitrate

These are dissolved in water, then evaporated until dry. 3 parts hydrochloric acid HCl are added and the solution is boiled. The gold is suspended in the simmering mixture, agitated and checked until the desired color is achieved. It is then washed in hot and cold water. the surface is partially dissolved in the solution with the alloy additions (the copper and silver components of the alloy) remaining in solution and part of the gold chemically plating onto the surface (Hebing, p70).

For golds with between 500 and 800 parts gold:

• 115 grams Sodium chloride
• 230 grams Potassium Nitrate
• 150 grams Water

While heating and stirring add 170 grams hydrochloric acid. The finished solution is boiled for one minute. The finished object is cleaned, rinsed and heated somewhat (until it oxidizes black) before using the solution to depletion gild it. The object is dipped in the boiling solution and agitated in it for approximately three minutes (Brepohl, p 315).

For improving the color of a lightly (mercury gilded) gilded article:

• 1 part Sulfur
• 1 part Potassium Bitartrate (Cream of Tartar)
• 1 part Sodium Chloride
• 0.5 parts Ginseng

Obtain a child's or a youth's urine and while still warm pour it in a dish and clean the grease from the gold work by painting with the urine. Put the mixture in a large copper or ceramic pot filled with boiling water, hang the work by a wire in the boiling mixture for one Ave Maria (courtesy of a Calgary priest: an average of three was 14 seconds), take it out and rinse. Repeat up to three times, more may blacken the work and damage the surface of the gold (Cellini, p 93).

Combine as powders:

• 115 g Sodium Chloride NaCl
• 230 g Potassium Nitrate KNO₃
• Add: 170 g Hydrochloric acid HCl

Heat until the mixture boils, hang the gold alloy object in the solution from a (platinum or fine silver wire, today one might use a titanium wire) wire for 3-5 minutes, rinse, repeat as necessary (Diebener, p 29).

For every 100 grams of alloy material:

• 100 g Potassium Nitrate KNO₃
• 65 g Sodium Chloride NaCl

Mix with water to form a thin paste, add:

• 8 g Hydrochloric acid HCl

Boil until one smells chlorine (This as most of these recipes is definitely not a safe procedure) and hang the work on a fine silver wire, moving it in the mixture for 1 and a half minutes, rinse, dilute the mixture with a little water, repeat for 2 minutes. Neutralize in dilute ammonia. This is suggested for material over 50% gold. The solution should be used only once (Diebener, p 29).

In Japan gold coinage alloys sometimes contained a fair amount of silver, enough that the surface of the metal was white and required depletion gilding treatment. They were painted with a mixture of iron sulfates, copper sulfate, potassium nitrate and calcined sodium chloride mixed with powdered resin into a water based paste. This was applied to the metal surfaces, the objects heated on a grate over a charcoal fire and then immersed in a strong brine solution, washed in water and dried (Hickman, p 74).

Other Salt Mixtures

An alchemist's procedure was as follows:

• 1 ounce Ammonium Chloride
• 1 ounce Potassium Nitrate
• 1 ounce Ferrous Sulfate
• 1/4 ounce Antimony (Extremely Toxic)
• 1 ounce Sulfur
• 1 ounce Charcoal Powder

Grind them all to a powder and add the amount desired to the melted high copper content alloy. Let it stand for fifteen minutes (at melt heat) and then pour almost all the copper out of the crucible leaving 8 to 10% of the original amount in the crucible. This is where the gold remains and it is finally separated using cupellation (Ganzenmüller pp 66-67). This procedure seems to me to be potentially very wasteful but is a good example of such an alchemical procedure. Note that the above mixture would definitely remove silver and copper from an gold alloy.

For coloring (depletion gilding) a thick layer of fire gilding:

• 1.5 parts red ochre (haematite)
• 1 part Copper Acetate
• 1 part Potassium Nitrate
• 1 part Ferrous Sulfate
• 1 part Ammonium Chloride

Mix well, grind finely adding with stirring using enough water to make a thick porridge-like mixture. Place in a large container as the mixture rises. A glass container is suggested. Silver is blackened with this mixture. Coat the gold parts of the object with the paste and place it on the fire. As soon as it smokes strongly quench in water. Do not let it completely smoke as it attacks the gold very strongly and can cause it to lose it's color (Cellini pa 93).

Another from Cellini is:

• 1/2 ounce Ferrous Sulfate
• 1/2 ounce Potassium Nitrate
• 1/4 ounce Ammonium chloride
• 1/2 ounce Copper Acetate

These are ground together beginning with the ammonium chloride. They are mixed with water to form a thick solution. It is heated on low heat and agitated for two 'Our Fathers' and then it is ready to use. The work is dried, coated with the mixture and put to heat. As the coating begins to smoke taking care that it not smoke (perhaps burn off) completely, the object is quenched in fresh water. It is cleaned and placed in boiling potassium bitartrate (cream of tartar) solution for an Ave Maria (14 seconds). It is then rinsed, cleaned and polished (Cellini, p 95).

Sometimes exposure over long periods of time to salts and corrosive chemicals in the earth may cause a depletion gilding effect. Maryon writes: "It has been observed that when objects made of gold have been buried in the earth for many centuries a change occurs in the metal near the surface of the work. Much of the alloying metal, be it copper, silver or some other metal is dissolved by the chemical action of the damp earth leaving a thin film of almost pure gold at the surface of the object "(Maryon, p 8). This observation, however logical seems to me not to take into account the possibility of the makers having deliberately performed a depletion gilding procedure on the metal.

Aldred notes that much Egyptian gold shows no difference in color between gold objects and their soldered joints. If welding or pressure welding was not used then the surfaces may have been deliberately etched using mild acids like vinegar to enrich them, or electrolytic action with salts in damp soils may have done the same (Aldred p 99). I had thought that most Egyptian gold was found in dry tombs and wonder if his observation is most likely due to deliberate depletion gilding.

Sometimes naturally occurring mixtures of earth and salts were used in a paste application to remove copper and silver from an alloy. Dampierre found in the Philippines in 1687 a natural gold-silver alloy used by the natives. When pieces made from it lost their gold color they were smeared with a red earth and placed in a fire, taken out red-hot and quenched in water, thus renewing the gold surface color. Probably the earth contained ferrous sulfate which breaks down to form sulfuric acid in the heat which removes silver from the surface. The resulting silver sulfate would be removed by polishing or even quenching (Ganzenmüller, p 42). As mentioned before on the Peruvian coast the clay used for this purpose contained a high proportion of potassium nitrate and sulfates of iron, potassium and copper (Tushingham, p 59).

Mixtures of salts have also been developed which attack one of the constituent metals in the three part alloy preferentially, so that if for example silver is preferentially removed the resulting surface may have the reddish tones of copper to it. One can then adjust an alloy's addition metal content and hence control it's color after having mixed it when alloying. Note that one might consider which starting alloy would be most advantageous for working with. Copper rich alloys are more easily attacked by acids than silver rich ones and might then be more useful for such surface color adjustment. High carat golds in general depletion gild better than lower carat ones (Brepohl, p 315). The work is dipped in the solution or it is brushed on to the work, and I assume then heated. The color of the surface may also be adjusted by pickling (etching) in HCl, H₂SO₄ or acetic acid. The mixtures described include:

To obtain a yellow

• 6 parts Potassium Nitrate KNO₃
• 2 parts Ferrous sulfate
• 1 part Zinc sulfate (this is a concentrated mixture)

To obtain a reddish-green

• 9 parts Copper Acetate
• 3 parts Ferrous Sulfate
• 3 parts Potassium Nitrate
• 3 parts Ammonium Chloride NH₄Cl
• 30 parts Water

To obtain a green

• 12 parts potassium Nitrate
• 4 parts Ferrous Sulfate
• 2 parts Zinc Sulfate
• 2 parts Alum
• 20 parts Water

>(Ganzenmüller, p404)

The pre-columbian Indians appear also to have used selective depletion gilding at times (Tushingham, p 60).

Wax Based Paste Depletion Gilding Methods

'Glühwachs' or annealing/heating wax was often mentioned in alchemical texts. The wax mixture which contained various chemicals would typically be spread over the object and then the object heated until the wax had burned off when it would be quenched. The process would be repeated as necessary. A mixture might contain wax, haematite and copper slag (copper salts or oxides). The wax removes dark copper oxides from the gold or from gilded articles reducing the surface to bright metal; 'reddens' the gold (Ganzenmüller p 70). The gold object is slowly warmed and coated with the wax mixture and is then heated until the wax burns and the flames go out. It is then often quenched in water, brass brushed with dilute acetic acid (vinegar), dried and polished.

Common ingredients are wax, copper acetate, iron oxide (haematite), zinc sulfate, copper oxides, ferrous sulfate and borax. Depending upon the mixture used one obtains a red or green tone. This depends upon whether silver or copper is more strongly attacked by the particular mixture and to some extent perhaps whether any plating action is taking place, for instance it is noted that copper salts in the wax may produce reddish tones on the gold. This might also be caused by reduction of the metal salt in the carbonizing (and reducing) atmosphere of the burning wax and possibly the charcoal fires used at the time. A brightening of the golden yellow tones of the surface colour may be obtained by having zinc sulfate in the wax and following with a scratch brushing with a very dilute Nitric acid solution (Ganzenmüller p 404).

For enriching the surface of mercury (fire) gilding: just heat to remove the mercury, then cool. Pickle lightly in a mild acid, warm over low heat so that one can apply the wax mixture to the metal, then allow it to cool.

• 5 ounces fresh wax (probably beeswax)
• 1/2 ounce Haematite
• 1/2 ounce Ferrous Sulfate
• 1/8 ounce copper sulfate
• 1/2 ounce copper acetate
• 1/8 ounce borax

Reheat the object on a fire until the wax is burnt off. Do not heat to red heat. Quench in a mixture of potassium bitartrate (cream of tartar) and water. Leave in the quenching solution for one Ave Maria (14 seconds), then brush and rinse, repickle if necessary (Cellini, p 94).

The Sulfur Process

This is best for high silver and low gold content alloys. The object is surrounded by sulfur and heated to turn the silver into black silver sulfide which eventually leaves the gold behind. In refining the silver alloy is granulated, heated with sulfur and the blackened (silver sulfide covered) granules are mixed with copper granules. Half of the metal is melted with an air blast. When it is molten the rest is added with a flux of litharge, lead granules, salt and Glassgalle. To this point I have been unable to find a correct translation for the latter. A gold alloy nugget remains and is melted with sulfur and copper granules. The procedure is repeated as necessary (Ganzenmüller pp 60-61). Sulfur and potassium nitrate with heat are used to convert copper to oxides, salts and sulfides to remove them from an alloy (Ganzenmüller p 66-67).

The sulfur depletion gilding and refining process may go back as far as the Egyptians. There is some thought that it may have been related to the use of Niello. A tenth century Arab dictionary describes how both copper and silver can be separated from gold when marcasite ( a sulfide) is added to the melt. Petrus Bonus reporting on refining in the Serbian silver mines in the 14th century writes: "In some silver mines one finds a very pure gold material…which does not appear to be gold but seems to be entirely silver. But during the refining which is done with sulfur both are separated from each other" (Ganzenmüller, p 60).

Theophilus describes separating gold from silver using sulfur. Pieces of sulfur are added to the melt "proportional to the amount" of silver and stirred with a charcoal stick until fuming ceases. Having done a similar procedure outside in making Niello I cannot recommend this procedure in the absence of a really good fume hood. I hate to imagine what it must have been like in a small, presumably closed in workshop space. As soon as the fuming ceases the metal is poured into an ingot mold. It is then gently hammered on an anvil to loosen the black silver sulfide layer (Theophilus, p 96).

Mineral Acid Depletion Gilding and Refining

Their use is somewhat more modern in approach as certain refined acids were available only relatively recently in historical terms. Nitric acid for example has been known at least since the thirteenth century (Ganzenmüller, p 62). In contemporary North America acids come in three main grades, Chemically Pure (100%), Reagent Grade (about 70%) and Commercial Grade (about 50%) (McCreight, p 30). The use of pure mineral acids, often hot presents a real ventilation and safety problem. A report from France around the end of the fifteenth century describes a gold refiner using "an artificial water" (Nitric acid) to refine gold. The report notes that he would often hire a stranger to perform the procedure and observe it himself from a distance. This says much about safety hazards and also perhaps hiring practices of the time. Experience has shown that a certain amount of silver in a standard alloy will not be removed by the nitric acid unless the gold is first alloyed to a proportion of 1 part gold (or less) to 3 parts silver when all the silver can be removed by the nitric acid. (Ganzenmüller, p 63).

Nitric acid, HNO3 is mixed 1:1 with water (Acid to Water: Always!). It is most effective with alloys of less than 25% gold content. If the alloy is copper rich then more copper is alloyed in before treatment. If the alloy is gold rich then silver is alloyed in. The gold present is untouched (Ganzenmüller, p 345). The procedure works better with copper-gold alloys than silver ones. For silver containing alloys one begins the process using weak acids and then adds stronger ones to avoid the separation of the gold as a colloid in the solution and it's suspension and loss (Ganzenmüller p 64). The chemical process as far as copper in the alloy is concerned is as follows (a similar process occurs with silver):

3Cu + 2HNO₃ => 3CuO + H₂O + 2NO (NO + O => NO₂)

3CuO + 6 HNO₃ => 2Cu(NO₃)₂ + 3H₂O

or
3Cu + 8HNO₃ => 3Cu(NO₃)₂ + 4H₂O + 2NO (=>NO₂)
(Brepohl, p 80)

In the preparation of gold alloys for enamelling the gold surface is often enriched as enamelling is easier and if transparents are involved, looks better over very high carat or pure gold surfaces. The object is heated and quenched in:

• 1 part Nitric Acid
• 32 parts Water

This procedure is repeated with rinsings and scratch brushings between dips (Linick, p 351).

Concentrated sulfuric acid H₂SO₄ may be used in combination with heating for silver-gold alloys with more than 10% silver content. The process is termed affination. A maximum gold content of 30% is recommended. The sulfuric acid produces copper sulfate CuSO₄ and silver sulfate Ag₂SO₄ from those metals in the alloy. The Ag₂SO₄ is precipitated by the addition of cold sulfuric acid H₂SO₄. Two to three washings with acid removes the copper sulfate CuSO₄ and the procedure can leave almost pure gold (Ganzenmüller p 345). The chemical attack on copper by sulfuric acid is:

Cu + H₂SO₄ => CuO + SO₂ + H₂O

then
Cu + H₂SO₄ => CuSO₄ + H₂O

or
Cu + 2H₂SO₄ => CuSO₄ + SO₂ + 2H₂O

(Brepohl, p 81)

When three part alloys are heated, only the copper oxidizes and in a standard dilute sulfate based acid pickle only the oxides are dissolved. This leaves the silver behind. To dissolve a little silver with the copper oxides a sulfuric acid mixture is used:

• 1 part H₂SO₄
• 1 part distilled water

The solution is used at 80oC

(Brepohl, p 314).

For high copper content gold an object may be annealed to oxidize as much surface copper as possible and pickled in dilute sulfuric acid H₂SO₄. This is repeated as necessary. It is also possible to use a less dangerous acid, the jeweller's standard pickle called Sparex® which is Sodium Bisulfate and is sold in swimming pool supply stores as 'swimming pool acid'. It is dry and granular and presents less storage problems than a pure sulfuric acid as it is stored dry and mixed up with water when required.

A boiling solution of 60o Be sulfuric acid H₂SO₄ in a cast iron pot has been mentioned as a preliminary stage in a refining procedure (This sounds rather nasty and perhaps unnecessary) (Ganzenmüller, p 66).

Sometimes Hydrochloric acid HCl is used in various concentrations. This is what I was taught to do by a Norwegian goldsmith, Christian Gaudernak: Heat the object and quench into pure or 1:1 HCl and water. The fumes and splash hazard are to my mind too extreme and I now leave the object in a sealed container of cold 1:5 or even more dilute HCl for longer periods of time. Hebing suggests this procedure for three part alloys; that is annealing and quenching in dilute HCl (Hebing, p 20). One book suggests overnight immersion in an HCl solution (Linick, p 400).

Sometimes mixtures of acids were used to 'color the gold' as with the following:

• 1 quart (US measure) Water, H₂O
• 1 ounce Nitric Acid, HNO₃
• Several drops Sulfuric Acid, H₂SO₄

The object is repeatedly treated with this solution (Linick, p 356).

Another coloring mixture is:

• 1 part H₂SO₄
• 1 part HNO₃
• 2-4 parts distilled water

Use the mixture warm. Sometimes concentrated HCl or diluted Aqua Regia is used for high carat golds. Note that using HCl can cause tension related corrosion in an unannealed piece of work (Brepohl, p 315). this is especially true in low carat golds (Rapson p 60).

Aqua Regia is used for refining but it has been noted that green gold (high silver content) and some low carat white golds containing considerable amounts of silver can be almost impossible to dissolve even in Aqua Regia as an insoluble layer of silver chloride is formed which protects the surface from further attack. In order to dissolve them they must be alloyed with several times their own weight of copper or brass and made into shot or granules before treating them again with the Aqua Regia. Aqua Regia consists of 4 parts HCl to 1 part HNO₃ (Loewen, p 3).

A very strong mixture intended for dipping treatment only is:

• 31 g Hydrochloric acid HCl
• 10 g Nitric Acid HNO₃
• 20 g Sodium Chloride, NaCl
• 40 g Water, H₂O

(Diebener, p 29)

To improve the color of gold alloys silver and copper oxides in the surface are removed. The degree of etching is determined by the alloy constituents. For gold-copper alloys:

• 1 part Sulfuric Acid
• 1 part Water

For gold-copper-silver alloys:

• 1 part Sulfuric Acid
• 1 part Nitric Acid
• 1 part Water

or

• 1 part Nitric Acid
• 1 part Water

or

• 30 grams Hydrochloric Acid (concentrated)
• 10 grams Nitric Acid
• 20 grams Sodium Chloride
• 400 grams Water

or

• 17 grams Hydrochloric Acid (concentrated)
• 11.5 grams Sodium Chloride
• 23 grams Sodium Nitrate (NaNO₃)
• 15 Water

or

• For gold poor alloys:
• Boiling Sulfuric Acid solution and 40 grams per liter nitric acid

or

• 25 grams potassium nitrate and a little Sodium Chloride

or For gold rich alloys:

• 200 grams Potassium Nitrate
• 100 grams Hydrochloric Acid (D=1.16)
• 100 grams Sodium Chloride
• 50 grams Water

or

• 200 grams Potassium Nitrate
• 150 grams Hydrochloric Acid (D=1.124)
• 100 grams Sodium Chloride
  (Ganzenmüller, p 403)

or

• 1 part Nitric Acid
• 1 part Sulfuric Acid

(Ganzenmüller p 28) Low carat gold alloys may be etched rapidly with:

• 1 gallon (US measure) Sulfuric Acid
• 2 pounds Zinc Oxide
• 1/2 ounce Potassium Dichromate

The mixture is used at room temperature. Additions of nitric acid increase the action (Linick, p 18-19). Potassium dichromate permits an oxidizing action on copper present in the solution which then allows the sulfuric acid to dissolve the copper oxides, as copper itself is not much affected by that acid. Sometimes pickles with sulfuric and potassium dichromate are used to remove copper 'flashing', a thin plating of copper that occurs on brass. An easier and less noxious approach to the same problem is to use a mixture of 50% sparex® solution and 50% hydrogen peroxide solution from the drug store. This removes copper flashing on work almost immediately and acts in the same manner, oxidizing the copper present so that the acid part of the solution may remove it. This solution too would work well in removing copper from the surface of a copper-gold alloy in combination with repeated oxidation of the metal surface.

An etching solution used for copper alloys that will also (in my experience) remove base metals (primarily copper) from gold alloys is:

• 480 grams FeCl₃
• 120 ml HCL
• Water to the 1 liter level

(from a period of study in West Germany, 1980)

Vegetable Acid Use

De Oviedo Valdes writes that the Indians understood how to gild low gold content work so that it appeared to be 23 or 24K, using 'certain herbs' (Ganzenmüller P 41). A plant the Indians used was Oxalis Pubescens which has juice that contains oxalic acid. Surface copper is removed by the acid to form oxalis acidic copper leaving the gold behind (Ganzenmüller, p 42). The plants were chewed and spat into a container to form the solution (Root p 253).

Preferential Electrolysis

"If two metals in a solution have a decomposition voltage far apart from each other, one may be almost completely separated from the other by plating it out at a voltage between the two decomposition voltages which at best would be just below the voltage required to plate out the higher metal" (Linick, p 407-408). This may also has some possible implications for potential safer techniques of removing mercury from an amalgam in submerged mercury gilding procedures.

A fine gold layer can be produced on a gold alloy object by using it as an anode in diluted acids: the base metals are dissolved out and the work is afterwards burnished. A 2% Nitric Acid HNO₃ solution works well with higher gold contents while it may produce a somewhat spongy surface in lower carats. An example of a rather toxic and dangerous solution used in industry for depletion gilding (bombing) with current is:

• 2 grams AuCl₃
• 16 grams KCN
• 4 grams KOH
• 4 grams Sodium Phosphate
• 1 liter distilled water

>(Ganzenmüller, p 406)

Resists to Depletion gilding

Depending upon the procedure used resists of various types can be used to preferentially depletion gild sections or parts of an object, usually as a method of obtaining patterned color variations on the surface. Relatively cool, wet procedures like electrolysis and immersion dips may be used with waxes, thinned rubber cement (my favorite for involving the least clean up), paints, asphaltum and so on, whatever can be applied, dried and is inert to the depletion gilding medium. For hot applications like a salt mixture or even in some cases a wax based mixture an inert material is applied to the surface before treating it with the corrosive mixture.

An example intended to be used with a salt (1 part), Potassium Nitrate (1 parts) and alum (1 part) mixture applied as a paste with a little water is as follows. Whiting, gum (gum tragacanth perhaps) and sugar are dissolved in water to make a thick paste. This is applied where the surface should not be enriched. It is partially carbonized and dried to a brown color. The exposed areas are then coated with the corrosive mixture and it is heated until it glasses over. Then the work is quenched into cold water, dipped in dilute nitric acid and rinsed again (Untracht p675).

There are without doubt a number of similar mixtures used based on inert media. I suspect that Liquid Paper® and similar typing correction fluid products might be effective in this regard. In 1987 the Papermate company ran evaporation rate tests on the material upon my request.for safety information on it. They informed me that it contains as a solvent 111 trichloroethylene which upon heating breaks down to form hydrogen chloride, chlorine and phosgene gas. They found that the material skins over rapidly and its evaporation rate slows down. Therefore if using Liquid Paper® I would recommend either the water based solution or waiting at least 20 minutes after application before applying heat.

Chemical Plating

Chemical plating occurs in some of the mixtures mentioned earlier in this compilation, especially where gold chloride is formed as a reaction product during the depletion gilding procedure. There are however many solutions devised specifically for contact or chemical plating, that is plating without an externally applied electric current. Some versions follow:

• 2 ounces Potassium Ferrocyanide
• 1 ounce potassium Carbonate
• 1 1/2 ounce Sodium Chloride
• 1 Quart (British measure) Water

The mixture is boiled in an enamelled saucepan, adding the salts one by one, stirring well with a glass rod. The solution is boiled for two to three minutes then 2 pennyweights (1/10th of an ounce) of gold chloride are dissolved in a little water (and presumably added to it). The solution is cooled and stoppered. To use it some of the liquid is heated to near boiling and the well cleaned article is placed on a piece of clean zinc and immersed in the solution for plating (Wilson, p 471). In this case the zinc is lower on the electromotive scale than gold and a displacement of ions occurs which causes the gold to plate out on the work.

One can apparently dissolve gold in "menstruum sine strepitu", a water based solution of salt, alum and saltpeter (which presumably forms gold chloride), soaks an linen cloth in the solution, burns it and uses the resulting ground up powder for gilding. It is noted that the procedure results in considerable losses in gold when compared to mercury gilding-though it is somewhat safer (Ganzenmüller, p 75).

Soak linen rags in gold chloride, dry and burn them, preserving the ashes. Clean the object to be gilded, rub over the work with damp leather until the gold color appears and then burnish (Wilson, p 239). I understand that the resulting plate is extremely thin and wears off rapidly.

So-called Greek gilding for copper alloys is made by dissolving equal parts of ammonium chloride and mercuric chloride in strong Nitric acid. With this mixture make a solution of fine gold and concentrate the solution by evaporation. The cleaned, pickled object is dipped in the solution or it is painted on. The surface turns black instantly if it is strong enough. It is heated to red heat and the gold then appears (Wilson, p 471).

For copper and copper alloy objects including copper plated steel the objects (including copper plated steel) are dipped in a boiling mixture of gold chloride with the addition of sodium bicarbonate, rinsed and polished (Hebing, p 29). Hebing also notes that an electrolytic bath has salt added to it and is then heated. The object to be gilded is suspended in the solution. A piece of zinc in contact with the solution will cause gold to be plated out on the object in a similar manner to electrolytic plating (Hebing, p 28). Other procedures include a solution of gold chloride and potassium cyanide (Really noxious materials) being mixed with chalk and placed on the object to gild it. A note on applying the gold chloride soaked ashes is that it works better if applied to the object to be gilded with a large singed cork which has been dipped in Nitric acid. After application the surface is burnished (Hebing, p 29).

Odds and Ends

Filed gold ground up with strong vinegar and a grain of salt with a carat (0.2 of a gram today) of sodium carbonate gives a contact color for gold illumination (Ganzenmüller, p 76).

A method of gilding mentioned in the Leidensis papyrus is to grind gold and lead as finely as wheat flour using 2 parts lead to one part of gold. The powder is mixed with gum and the ring or object is coated. It is then heated and repeated several times until the desired color appears. As it is heated the lead melts away leaving the gold behind. The object is difficult to test as it leaves a gold streak on the touchstone (Ganzenmüller, p38).

While not contact gilding as such sometimes fine abrasives (pumice, glass, powdered quartz were introduced into parchment and gold rubbed onto it to color it. This is related to gold point and silver point where clay particles are incorporated into paper so that a gold or silver wire is abraded and leaves metal on the surface of the paper (Ganzenmüller, p 77). Gold and silver wires have long been used to draw on clay impregnated papers (silverpoint and gold point) and soft gold has even been used to decorate ceramics by rubbing directly on them. This concludes this listing of procedures. I hope that it has been of some interest and while it is by no means complete in any sense I would hope that it gives a feeling for and through contrast and comparison lends an understanding of how goldsmiths have attempted to deal with depletion gilding.

Bibliography

• Aldred, Cyril, Jewels of the Pharaohs, Praeger Pub., NY, Washington, 1971
• Brepohl, Erhard, Theorie und Praxis des Goldschmiedes, VEB Fachbuchverlag Leipzig, 5th Edition, 1978
• Bray, Warwick, 'Ancient American metallurgy: Five Hundred Years of Sudy', p 76-84, The Art of Precolumbian Gold, Edited by Julie Jones, Little, Brown and Company, New York, 1985
• Cellini, Benevenuto, Abhandlungen über die Goldschmiedkunst und die Bildhauerei, Ubersetzung: Frölich, Ruth und Max, Gewerbemuseum Basel, Basel, Switzerland, 1974
• Diebeners, Wilhelm, Werkstattrezepte für Graveure, Gürtler, Galvaniseure und Stempelhersteller, Wilhelm Diebener, Leipzig 05, Druck; Glass und Tuscher, M 135-305
• Emmerich, Andre, Sweat of the Sun and Tears of the Moon, University of Washington Press, Seattle, 1965
• Ganzenmüller, Wilhelm, Gmelins Handbuch der Anorganischen Chemie, System Nummer 62, GOLD, Lieferung 1 und 2, Verlag Chemie GMBH, Weinbaum, 1950
• Hebing, Cornelius, Vergolden und Bronzieren, Verlag Georg D. W. Callwey, München, 1960, 14th Printing 1985
• Hickman, B. Editor, Japanese Crafts, Materials and their Applications, Fine Books Oriental, London, 1977
• Linnick, Leslie, Jewelers Workshop Practices, Henry Paulsen Co, Chicago, 1948
• Maryon, Herbert, Metalwork and Enamelling, Dover, NY, 1971, republication of 1959 edition of Chapman and Hall, London, 1st Edition, 1912
• McCreight, Tim, The Complete Metalsmith, Davis Pub, Worcester, Mass, 1982
• Morton, Contemporary Jewelry, Holt, Rinehart and Winston, New York, 2nd Edition, 1976
• Plazas, Clemencia and Falchetti de Saenz, Anna Maria, "Technology of Ancient Columbian Gold", Natural History, 88, No. 9, 37, 1979
• Rapson, W. S. and Groenwald, T., Gold Usage, Academic Press, London, New York, San Francisco, 1978
• Root, William, "Pre-Columbian Metalwork of Colombia and it's Neighbors", Essays in Pre-Columbian Art and Archeology, Harvard University Press, Cambridge, Mass, 1961
• Seeler, Margaret, The Art of Enamelling, Galahad Books, New York City, 1969
• Theophilus, On Divers Arts, Dover Publications, New York, 1963, 1969
• Tushingham, A. D., Gold for the Gods, Royal Ontario Museum, Toronto, 1976
• Untracht, Oppi, Jewelry Concepts and Technology, Revised Edition, Chilton book Company, 1972
• Von Neumann, Robert, The Design and Creation of Jewelry, Revised Edition, Chilton Book company, 1972
• Wilson, H, Silverwork and Jewellery, John Hogg Pub, London, 1912