Gaffer Glass

Casting Glass

Red Wedge 550mm tall
www.richardwhiteley.com

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:

An ability to achieve the entire colour spectrum with the same base glass. This would allow firing and casting characteristics to be the same across the range. Colour could then be mixed.
Extreme resistance to devitrification.
Low viscosity at casting temperatures.
Low annealing and strain point temperatures.
High surface definition.
A high refractive index, high dispersion and good optical clarity.
Excellent cold working properties.

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

Billets.

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 800-900^oC. (1470-1650^oF), 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.

Frit.

SPECIFICATIONS:
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): 440°C. (824°F)
Strain point: 400°C. (752°F)

Casting Crystal Lead Emissions.

Several customers over the years have asked us whether kiln firing our casting crystal at top melting temperatures gives off harmful lead emissions.

In order to give an accurate scientific answer to that question we commissioned in 2001 a laboratory report from Watercare Services Ltd in Auckland, a company employed by the City Council to monitor water and air pollution. The purpose of the study was to assess compliance with Occupational Safety and Health (OSH) Workplace Exposure Standards (WES) and Biological Exposure Indices for New Zealand (2001)

In order to give worst case results the lead monitoring was carried out in the studio of local artist Anne Robinson, melting 2 x 25kgs of casting crystal at 870°C (1600°F) in a deliberately unventilated room. All doors and windows were shut. The methodology for determining the lead in the kiln room was as follows:

Methodology

A sample of ambient workplace air (next to the kiln vent) was drawn through an SKC IOM Inhalable Dust Sampler (plastic) to capture particulate lead. The IOM sampler is designed to capture dust, which is fine enough to be inhaled by employees. The sample gas stream was then passed through a dilute solution of nitric acid and hydrogen peroxide. This solution captures any lead fumes, which may pass through the filter. An SKC Personal Sampling Pump calibrated to a flow rate of 2.00 ± 0.01 litres/min was used to draw sample through the sampling system.

Results and Discussion

The results of the lead monitoring are presented in the Table below:

Run Site	Sample Time (Mins)	Volume sampled (m³)*	Mass (mg)	*Concentration (mg/m³)TWA**	*Concentration(µg/m³) TWA**
Kiln Room	240	0.480	0.009	0.02	19
Kiln Room	180	0.360	0.004	0.01	11
Adjoining room	240	0.480	0.007	0.015	15

*Corrected to 25^oC and 101.3 kPa

The current NZ OSH WES for lead is 0.1 mg/m³ and the current USA OSHA-PEL for lead is 50µg/m³. Reviewing the data presented in Table 1 it can be seen that in the worst case the level observed is approximately at worst 5 times below the OSH guideline and approximately 3 times below the USA OSHA guideline.