Firing silicone mastic

Several people have asked over time about the consequences of firing glass with some silicone mastic (or caulking) still attached to the glass. 

I can say with confidence that it does not break the glass.

I can also say, that it really is not a good idea.  Take as much as possible off with knives, then use silicone disolvers to remove the remainder.  These photos show the results of firing silicone residue.

Where there were pieces of silicone, a divot appeared with the black combustion product from the mastic/caulking.  Where there were strips of silicone, a small valley occurred.  The smallest amount of silicone appeared as a dark divot in the glass.  

It is possible to remove the silicone residue with sandblasting.  Other abrasive methods are possible, but much more time consuming.   Once the silicone has been sandblasted away, the glass needs to be cleaned of all the dusts, and covered with a fine layer of fine frit, or if prefered, powder.  But I find fine frit works better, although it requires a full fuse to form a smooth surface.

Glass Dust

This is from Greg Rawls' website.  He is a glass worker and a certified industrial hygienist. A huge amount of practical information on safety in glass working is available on his web site:

http://www.gregorieglass.com/Health_Saf ... mical.html

Ground Glass

OSHA classifies glass dust as a “Nuisance Dust”. Ground glass does not cause silicosis. You can wear a respirator if you are concerned about exposure.

Glass is made from sand, which contains silica - a naturally occurring mineral silicon dioxide (SiO2). Crystalline forms of silica, also known as “free” silica, can contribute to the development of silicosis under prolonged exposure conditions.

It is important to understand the difference between glass and crystalline silica because exposure outcomes are extremely different! Glass is a silicate containing various other ingredients which have been melted and upon cooling form an amorphous, or non-crystalline structure. While silica (SiO2) is a primary ingredient in the manufacturing of glass, when glass is formed under heat, the crystalline structure is changed to an amorphous structure and is no longer considered crystalline.

Ground glass is rarely respirable because the particle is too big. Always use wet methods when grinding glass! Water captures the dust. Sometime other chemicals are used to add colour to glass such as arsenic, lead, cadmium. These are usually present in low concentrations and are bound to the glass and not readily available but could present an exposure issue under some circumstances.