Enamel is Glass — But, What is Glass?

In the beginning, there was glass. Glass was created or formed along with our planet Earth. Glass was here — waiting to be discovered by humankind, as early as 75,000 BC, to use as spear points, daggers and crude axes. We are referring, of course, to natural glass rather than human made glass.

Volcanoes emit lava, which may cool as a glass (obsidian) or crystallize into rock masses, depending on composition and rate of cooling. In Yellowstone National Park, there is a great mass of volcanic glass, nine miles long and five miles wide, in which there are only occasional traces of crystalline forms. When volcanic glass is impregnated with steam, it forms a glass of cellular character known as pumice. In the islands of Lipari in the Mediterranean, there are two mountains of pure pumice.

Other natural glasses are "fulgurites" and "tektites". Fulgurites are crooked tubes of fused silica glass, imperfectly melted, often several feet in length and sometimes forked, formed by lighting when it strikes sandy ground. Lumps of high silica glass which have fallen from the sky are found in various regions of the earth. They are called "tektites", generally black and opaque. There are two schools of thought as to their origin. One group maintains they have been formed as molten debris by meteorite impacts and scattered widely through the air. The second group believe they have been formed by volcanic eruption on the moon. There are many known falls spanning millions of years. Those found in North America are thought to be about 34 million years old. Soil brought back from the moon by astronauts contains a high proportion of glass, mostly in the form of tiny spheres. Glass may play a more important role in the universe than we know.

The abundance of elements in the earth's crust is given in the CRC Handbook of Chemistry and Physics. From this information, an average oxide composition can be calculated in percentage by weight:

• SiO₂ - 59.1
• Na₂O - 3.7
• K₂O - 3.1
• CaO - 5.1
• MgO - 2.7
• BaO - 0.1
• Al₂O₃- 15.3
• TiO₂ - 1.0
• P₂O₅ - 0.3
• FeO - 7.3
• MnO₂ - 0.1
• Other - 2.2

A batch of minerals selected to provide this composition was smelted at 2600°F, forming a viscous liquid. It was cooled quickly by pouring into water, forming a black glass. Thin sections showed some transparency, but additional thickness quickly produced opacity. It appeared unsuitable for normal glass working procedures due to its tendency to devitrify rather easily. Obviously, if our earth gets too close to the sun, we may be part of a rather sizeable glass marble.

There are many unanswered questions about who first tried to replicate natural glass. What sparked the idea and how did they go about it? What led them to combining a siliceous material with ashes? An often quoted answer was written by Pliny the Elder in 77 AD. A group of sailors building a fire on a salty beach, supporting their cooking pots with cakes of soda and, "a strange translucent liquid flowed forth in a stream." Subsequent writers have suggested glass making originated from the production of faience or as a 'slaggy' byproduct of metallurgical processes. Faience is a ceramic body composed predominantly of crushed quartz and covered with an alkaline glaze. Faience has been produced in Egypt and Mesopotamia from about 4000 BC, until modern time. The metallurgical source would have been copper smelting, which dates from about the same time.

None of these hypotheses leads us to ashes. Perhaps we should look at the oldest pyrotechnology. Small ceramic sculptures of both humans and animals, fired to perhaps only 600-800°C (1112-1472°F), dating from 23,000 BC, are the earliest fired clay objects found to date. In the later Stone Age (c. 10,000 BC), ceramics advanced into the realm of fired pottery vessels, and eventually iron-rich clays, already long familiar as pigments for paints, proved ideal for creating permanent fired designs on hand-built pots.1 Page 18 of this reference indicates early Bronze Age potters in China observed areas of natural ash-glaze developing, both on the pots and on the kiln walls. The hot wood ashes were carried through the kiln by the draft of the fire, some dropping on the pots and some sticking to the walls. Observations of this effect may have lead to China's first experiments with applied glazes. This phenomenon may have been observed thousands of times, in various parts of the world. Perhaps an area of only one inch in diameter was enough to cause an inquisitive person to experiment.

All four hypotheses may have produced glass, useable or unusable, under the right circumstances. Again, none of the four may have had any influence at all. There is not enough evidence for a definite answer.

We do know mixing ashes with a predominately silica material worked. Fulgurites, mentioned above, demonstrated glass could be formed by heating silica to a very high temperature. And we know ancients used a mixture of silica and ashes to produce glass at lower temperatures. Their usual formula was three parts ashes and one part silica, both by weight. As an alternative, two parts silica and one part purified ashes.

Ashes were purified by sifting through a fine sieve and adding to boiling water. Boiling continued, with stirring, until one third of the water had evaporated. Water was added to replace that which had evaporated. Boiling continued until two thirds of the water evaporated. At this time, the fire was slackened and the contents - ashes and water - were ladled into earthen bowls to stand for 10 days. The clear liquid was ladled into other bowls and let stand for two days. The liquid was placed into a container and boiled softly until it thickened. The contents, lye, was removed and dried.

Ashes, like glass, is not a stoichiometric material. The compositions of both vary widely. By comparison, common salt is a stoichiometric material — always one atom of sodium combined with one atom of chlorine. Since ashes vary in composition, some are more suitable for glass making than others.

Robert H. Brill' discussed in detail the ashes used in ancient Mesopotamia in the first and second millennia BC. The chianan plant mentioned in Cuneiform texts still grows in the Syrian desert. In 1956, W.E.S. Turners published the composition of Keli or ash from the Syrian Desert chianan plant, taken in 1943:

Parts soluble in water 60%

• 45% Sodium carbonate
• 2.5% Sodium hydroxide
• 4.5% Potassium chloride
• 3.0% Potassium sulphate
• 5.0% Other

Parts insoluble in water 40%

• 34% Calcium carbonate
• 4% Calcium phosphate
• 1% Magnesium carbonate
• 1% Carbon

Brill recalculated the above composition for easy comparison with samples of the chianan plant taken in 1969:

The 1969 ashes were from thin stems of the chianan plant growing in the Syrian Desert. When making the ashes, a lower temperature than usual was required. The normal procedure was a temperature of 750°C to ash plants. The stems fused completely at this temperature, apparently forming a molten lye. A second sample was asked at 600°C.

It would appear both compositions of keli would be suitable for making glass using the three parts ash and one part silica sand recipe. They would not require purification. This is not the story with the Kentucky tree ashes. These ashes resulted from burning, in an open fire, fallen dead limbs from oak, elm, dogwood and miscellaneous trees growing on a rocky hilltop on the writer's property in Northern Kentucky. We found it impossible to make glass using three (even more) parts ash and one part silica. However, the two parts silica and one part purified ashes produced a fairly workable glass when using the normal industrial-grade silica we use to manufacture enamel at Thompson. The culprit, of course, was the high calcium content which is insoluble in water, and was elimintated by the purification procedure. When substituting finely ground rocks found on my rocky hilltop for the industrial-grade silica, a billous greenish-brown glass was produced. It was hardly workable with a fusing temperature close to 1700°F. We should have had the rock analyzed to see what was present other than silica. At the moment, we were satisfied having made glass using only materials from our rocky hillside.

References:

1. Wood, Nigel. Chinese Glazes. A&C Black, Ltd. London. 1999. P. 11.
2. A.L. Oppenheim, R.H. Brill, D. Barag, & A. Von Saldern. Glass and Glassmaking In Ancient Mesopotamia. The Corning Museum of Glass Press. Corning, NY. Reprinted 1988. P. 124.
3. W.E.S. Turner. Journal of the Societies of Glass Technology. Vol. XL. June 1956. pp. 277T-300T.

By Woodrow Carpenter

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© Glass On Metal – Vol 27 No 2, 2008 April

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