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Casting - Instructions for Gold Ruby #221 & Amethyst #276

Gold Ruby belongs to a group of glasses where the coloration depends on reheating the glass to a certain viscosity in order to cause the metal atoms to form a colloid and at a certain particle size transmit color. Copper and Silver are other noble metals that exhibit the same behavior as Gold. The color development process is called “striking”.

Our Gold Ruby #221 and Gold Amethyst #276 casting crystal is specially designed to be “struck” under casting conditions. Gold ruby #221 comes in a very pale pink state. (It is doped with a small amount of Erbium to distinguish the glass from Clear #210). The Gold amethyst #276 shows the amount of cobalt blue in the glass that will turn the ruby into an amethyst. If we struck the glass before on selling it, then it would become “overstruck” under the extra heating regime of casting.

This glass requires a reasonable amount of observation and control by the end user to achieve the right shade. Conditions will vary between different kilns, molds, and the shape and size of the work. It is recommended that small-scale experiments are carried out until confidence is gained to make larger scale work.

The ideal ruby color is attained when many small gold particles are formed, being neither too small, nor too large. The evidence is in the color. A very small number and size of gold particles will color the glass firstly a pale brown and then with time, the brown gives way to a purple shade. Continued heating slowly increases the size and number until a ruby eventuates. If the Gold Amethyst is arrested at the purple stage the color may be too deep a purple. Longer firing will lighten the hue. If the heat is too high, or too prolonged, then a color phenomenon results called Sapphirin. This has a livery brown aspect on the surface in reflected light. The ruby then has been spoiled because the many small colloidal particles of gold have adhered to each other and grown too large.

Our recommendation is to heat the ruby up from cold either in the mold or in a reservoir. The best rubies like slow heating. Charging the mold with cold cullet at casting temps of 750-850oC (1380-1560oF) will encourage the growth of a few large particles rather than many small ones, and the result will be liverish.

Do not fire the crystal as hot as you might with the other colors. Try running the glass in at around 780oC (1450oF) and when you are satisfied the glass is fully melted into the mold, drop the temperature to around 700-710oC (1290-1310oF). Hold it there for about 3-4 hours before dropping down to the annealing temperature range. It is a good idea to have a small sample you can take out of the kiln and inspect while you are firing to be sure the color progression is moving through the purple range and into ruby. This also guards against overfiring the piece as well. Try to avoid the surface of the glass being exposed to direct radiation from the electric elements as this may cause the surface to become overheated and become livery.

Be prepared for the intensity and hue of the ruby to change from firing to firing unless firing times and temperatures are strictly repeated. Dilution with clear or mixing with other colors may change striking rates.

There is a limit as to how much the ruby can be watered down by clear. Don’t attempt more than 50/50.

Also the ruby is very sensitive to reduction agents that may be present in other colors. Unpredictable results may come from mixing it with #’s 220, 230, 240, 242, 245.

Gaffer® Coloured Glass Ltd. 2000

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