Minerals such as silica, soda ash, potassium nitrate, borax, calcium carbonate, etc. are weighed according to each enamel formula, well mixed, and loaded into a preheated fire clay crucible. The batch is heated to a selected temperature for a sufficient length of time to melt the minerals, forming a viscous liquid, and continued until all gases released during the decomposition of the minerals are eliminated.
The enamel is removed by ladling or pouring it onto a thick iron plate where it cools to form “cake” or “lump,” into water where it is shattered to be called “frit,” or through water cooled iron rolls to be called “roll quenched.” The cake and roll quenched are also known as air cooled. The minerals mentioned above will produce colorless transparent enamels. Changing the proportions of these minerals will provide a wide range of firing temperatures and thermal expansions, as well as small changes in gloss, surface tension, and other properties less well known.
Other minerals may be included in the colorless transparent enamel formula. Some crystalline minerals have a low solubility in glass. These insoluble crystals will decrease the transparency. Depending on their size, number, and index of refraction, the decrease can range from a slight cloudiness to a dense opaque white. This range of opacity has been described in terms such as translucent, opal, opalescent, and opaque.
Several manmade ceramic pigments are added to control color. When added in the absence of insoluble crystals, the result is a transparent color, frequently called translucent. When insoluble crystals are present, the result is an opaque color.
Lead Free and Lead Bearing Enamels
The composition of lead and lead-free enamels differ only in that the former may contain as much as fifty percent lead while the latter contains no lead. Any change in an enamel’s composition may require some change in the procedures used by the artist who wishes to obtain the same end results. Different enamel compositions have different optimum firing ranges to produce their best visual properties such as gloss, clarity, and smoothness. This firing range is rather narrow, usually a spread of 3050°F, and a length of time AT THAT temperature, which must be determined by the artist.
Furnaces that are slow coming back to temperature after inserting the piece may require a total of 5 minutes or more to provide the proper length of time at the optimum temperature. Artists who fire by observing the surface of the piece, with little regard to what their indicating pyrometer reads, have had no trouble firing lead-free enamels. Artists who arbitrarily fire at what they think their indicating pyrometer should read, and at a predetermined time, have problems switching from one enamel to another even if both are lead enamels.
I cannot tell you what your pyrometer should read or how long you should fire to obtain a clear flux coating of lead-free enamel on copper. I can tell you that in my furnace with my controlling pyrometer set at 1500°F the length of firing is four minutes. If you can arrive at a comparable fire, the result will be equal to or better than any lead-bearing enamel. When using opaque lead-free I set my controlling pyrometer at 1500°F and fire for three minutes. Using a controlling pyrometer eliminates the need to look at the enamel while in the furnace once you determine the optimum fire (temperature and time).
We know many artists who feel their lead-free transparent colors are superior to lead-bearing enamels. On the other hand, many feel lead-bearing opaques have a little more gloss, but they continue to use unleaded for health reasons.
Lead enamels can be fired as a subsequent coat over lead-free enamels; lead-free enamels can be fired as a subsequent coat over most lead enamels. They can be sandwiched. For example: a lead flux, lead-free color, followed by a lead color. Again, the secret is in the firing. All of this was done quite extensively in the sixteenth century in Limoges. Prior to that, the eleventh century champlevé enamels by the Limoges and Mosan masters were lead-bearing white, yellow, and green, while all other colors were lead-free.
There is one caution. As mentioned above, a complete coat of lead enamel can be applied over a fired coat of lead-free, however, if only one or two grains of lead enamel falls onto a lead-free surface and fired, a pit will result. Thompson stopped making lead enamels because of OSHA’s workplace restrictions on the airborne lead.