History and Facts About Enamels
Art Enameling on Metals by H.H. Cunynghame 1906
Metals such as gold, silver, iron, copper and fine brass; copper with a small amount of tin, are capable of being enameled. But nickel, zinc, or common brass, which contains a mixture of copper and zinc are not capable of being enameled. Mr. Cunynghame, first speaks of the art of enameling is simply melting glass on to the surfaces of various substances capable of adhering to it and also capable of withstanding the heat necessary to melt the glass and cause it to flow. Mr. Cunynghame, speaks of the common technology of producing Crockery, with is clays consisting of silicate of aluminia and magnesia, and forming it into shapes and firing it. After this process it is know as biscuit ware, and is hard and porous. At this point glass ground to an impalpable powder is mixed with water and the vessel dipped into the glass slurry, dried and fired to the melting point of the glass. In this respect the glass is called “glaze.” Very similar to our enameling and as well as the commercial applications of enamels or glass on other foundations such as iron= stoves and refrigerators or bathtubs. And don’t forget the use of enamels on signage.
Glass consist of sand and an alkali such as soda or potash, fused together with the addition of ingredients to produce color. He states the peculiar value here of lead. It gives the glass the remarkable ability to adhere to and run over the item being enameled and prevents it from cracking. He state it makes the glass more elastic and enhances its’ ability to withstand extreme temperature changes. He also says that glass without lead is not suitable for enameling, as it will crack away from the metal. Of course today we have found a way to produce enamels without lead. The replacement for the major part is borax.
Glass is colored by melting it with the oxides of various metals. Oxide of tin makes it opaque white, iron gives it a sea green and yellow, cobalt a royal blue, and copper a turquoise blue. Manganese colors glass violet, silver and antimony yellow, and gold crimson. Also we have chromium, producing a fine shade of green; uranium, a fine yellow; and iridium, giving enamels the shades of steel gray and black.
Coefficient of thermal expansion is when the temperature of a substance changes. The energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds. As a result, solids typically expand in response to heating and contract on cooling. This dimensional response to temperature change is express by its coefficient of thermal expansion. This is where many get lost.
In enameling you just need to match the expansion of your metal and your enamels. I use a harder firing enamel as a flux = base layer in the technique of cloisonne, , still in the family of medium expansion enamels but slightly harder. Then continue with medium firing enamels just a bit softer. For the very simple reason, with repeated firings the base flux layer is harder to flow, and the medium temp enamels flow or fuse faster. If the base layer melted at the same temperature and time as the later layers it would be possible for your warm colors to get liquid enough to mingle down through the base flux and eventually touch the metal and burn. We are using medium firing enamels, they are for copper, sliver and gold metals, but a few are a little bit harder than others and these should be laid down first.
If you make test plates you will see this. Below I have a plate 1″ x 3″ and I put 15 color dots of washed enamel on the plate and fire it at 1425 degrees. Some dots are completely fused and some are still grainy. The grainy one is a harder firing enamel and takes more time to fuse.
Another point here to help in the technique of plique a jour, as well as in cloisonne, you do not want to put a harder firing enamel next to a softer firing enamel, again you will have cracking. When cooling the less expansion enamel right next to the high expansion enamel do not mingle, and separate= cracks.In this piece I made for Andre of Outkast, he wanted the cloison wires to be fine silver and different thicknesses. Which caused a lot of heat and of course with a pink nose against the fine silver cloison wires was trouble. But in my pallet of colors my pinks were not of the same firing hardness, once the cracking started, I had to remove them and test to get ones that were all the same to get the cracking to stop. And I am not talking about layering these colors I was gradating the colors next to each other. Things like this teach you but it would have been nicer to know ahead of the project.
When you get into plique a jour you will find it is also important to know your enamels melting points as in grouping or shading. When laying colors next to each other with no backing it is more obvious if the colors do not blend together. When the grains do not mingle and with out the back foundation as some expansion unity it is more likely to have cracking.