This range of transparent coloured glass was designed to withstand the special demands of both large and small lost-wax and investment plaster casting applications. Properties that Gaffer® considered to be valuable for an all round casting glass included the following:
The challenge involved in meeting some of these goals is constrained by the temperature limitations of Plaster of Paris, or gypsum, the main ingredient of common glass casting moulds. Plaster of Paris has an upper temperature limitation of around 900°C. (1650°F). It begins to decompose at around 880-900°C. (1616-1650°F), both physically breaking down and losing strength, while at the same time giving off SO3 gas, which introduces surface blisters into the glass object. This makes the use of typical soda lime glasses, and even medium lead content glasses unwise, because of their strong tendency to devitrify at around 800-900oC. (1470-1650oF), owing principally to their relatively high viscosities at these temperatures, and a propensity for faster devitrification type crystal growth.
In addition soda lime glasses, in particular, generally have a considerably higher annealing point, which means longer cooling cycles are required, especially in the case of massive pieces.Gaffer® casting glass, however, has virtually no tendency to devitrify at all at top casting temperatures, nor on its descent in temperature down to the annealing range. Furthermore, its very low viscosity, and low surface tension at 780-900°C. (1426-1650°F), leads to very good surface definition. This makes it especially suitable for jewelry scale pieces, as well as large work. Low annealing temperatures, and superior cold working characteristics, lead to lower costs overall. The chemical formulation closely approximates glasses used by the Czech casting artists.Careful control of time/temperature melting parameters, along with special conditioning techniques ensures that all colours are homogenous and compatible with each other.
Linear expansion coefficient (a): 92x10-7 (20-300°C.)
Density: 3.6g/cc. (Or 2.08 ounces/cu.in. or 3.73 ounces/US fluid ounce).
Casting temperature (Recommended): 780-850°C. (1426-1562°F.)
Annealing temperature (tg): 430°C. (806°F)
Strain point: 390°C. (734°F)
Softening point: T107.6 594°C (1101°F)
Working point: T104 902°C (1655°F)
Refractive index (G-210): (nd) 1.620
Young’s modulus E: 8.27 x 106 psi (57 x 103 N/mm2 )
Glass weight equals 4.2 times the wax weight.