01/05/2014
Lampworking can be done with many types of glass, but the most common are soda-lime glass, sometimes called "soft glass," and borosilicate glass, often called "hard glass." Leaded glass tubing was commonly used in the manufacture of neon signs, although its use has been fading due to environmental concerns and health risks.
Soft glass is sometimes useful because it melts at lower temperatures, however it does not react well to temperature changes like borosilicate glass does. Because soft glass expands a lot more than hard glass when heated, and contracts more when cooled, along with its brittle nature, during cooling or even while working it, soft glass is more prone to cracking due to thermal shock than borosilicate. If a piece cools too quickly in one spot, such as if cold water touches a thin piece of soft glass, that part would solidify immediately, and cause a crack. However, hard glass or borosilicate, would be more resilient. Borosilicate is just like regular glass, SiO2, but it has a more flexible molecular structure from being doped with boron.
The colors of the glass must be selected for compatibility with each other, both chemically (more of a concern with soft glass than borosilicate) and in terms of coefficient of thermal expansion (COE) [CTE is also used for Coefficient of Thermal Expansion.] Glass with incompatible COE, mixed together, can create powerful stresses within a finished piece as it cools, cracking or violently shattering the piece. Major types of glass, e.g., borosilicate and soda-lime glass, are not compatible with each other. Chemically, some colors can react with each other when melted together. This may cause desirable effects in coloration, metallic sheen, or result in an aesthetically pleasing "web effect". It also can cause undesirable effects such as unattractive discoloration, bubbling, or devitrification.
Borosilicate glass is considered more forgiving to work with, as its lower COE makes it less apt to crack during flameworking than soda-lime glass. However, it has a narrower working temperature range than soda-lime glass, has fewer available colors, and is considerably more expensive. Also, its working range is at higher temperatures than soda-lime glass, requiring larger torches and the use of oxygen instead of air. In addition to producing a hotter flame, the use of pure oxygen allows more control over the flame's oxidizing or reducing properties, which is necessary because some coloring chemicals in borosilicate glass react with any remaining oxygen in the flame either to produce the desired final color or to discolor if extra oxygen is present.
