Glass Archive Organized by month.
July 2006 / August 2006 / September 2006 / October 2006 / November 2006 / December 2006 / January 2007 / February 2007 / March 2007 / April 2007 / May 2007 / June 2007 / July 2007 / August 2007 / January 2008 / July 2008
November 2010 / December 2010
Happy Halloween! So in the spirit of dress up, today we will talk about what to wear when your in the hot shop. To stay cool when you're in the furnace area make sure to wear long-sleeved shirts and long-sleeved pants. Why? Because they form a barrier that keeps the radiant heat of molten glass away from your skin.
Don't send a boy to do a man's work (and vice versa). Goblet jacks were not meant for huge pieces and you might find those jumbo jacks a bit unwieldy for small pieces.
Are there more scientific ways of testing for glass compatibility then putting it in the freezer?
Oh yes. The most common (with gaffers and lampworkers) is to heat a small amount of each glass and then pull them into a thin rod (cane). Once it cools, if the glasses are not compatible, the cane will no longer be straight. The one that expanded more will have pulled the other off center creating a slight curve to your rod. Of course, if they are, if they are really incompatible, the rod will break upon cooling.
Fusers don’t normally pull out threads of glass so for them to test compatibility they take a clear piece of glass they normally use (and probably know it’s COE Bullseye=90, Pyrex=30 etc) and place a small square of the glass of unknown COE on it. Fuse flat < 1500F. If the glasses are compatible they will nicely fuse into the base.
To get really scientific you can further test your melted rod or square by using polarized lenses. You can even use lenses out of polarized sunglasses unless want to go down to the camera shop for polarized film or a specialized piece of equipment. Sandwich the glass between the lenses; rotate them to right angles (blocking the light from getting through); hold the whole thing up to the light and see how much of a “halo” you can see around the edges; the greater the halo the greater the incompatibility.
Even if using glass of a “known” (“well, the saleslady said…”) COE, you may want to test it before you buy out the store.
How do you know if two types of glass are incompatible?
Test ‘em. There are several ways of doing so. The easiest way is to fuse a sample of 2 pieces together. If they have widely different COE’s they probably won’t make it out of the kiln still fused. But if they are close and have micro fissures that may fracture later or show up in bigger samples, you will want to know. Aunt Martha won’t appreciate it if the bowl you give here for Christmas suddenly shatters during dinner. So let the test piece return to room temperature and freeze it ( in a baggie or some container where it breaking won’t ruin your steaks). After an hour or so let it again return to room temperature. If it is still intact, the glasses are probably compatible enough to use.
When you find your glass has cracked, you had bet find out why lest you repeat the crime.
Obviously, glass incompatibility should be your #1 suspect. Different glass types can have different rates of expansion when they get hot. This is expressed in one of those weird mathematical logarithms as the COE (Coefficient of Thermal Expansion). A glass with a COE of 90 means and expansion or contraction rate of .000090 or 9 X 10-6 which is a lot more information than you need to keep your glass from cracking.
However, you have probably used varying COE’s to your advantage when trying to open the lid from a glass jar. Traditional wisdom is to hold the top of the jar under hot water. The metal lid expands more and faster than the glass, so it gets a wee bit larger and therefore makes opening the jar easier.
That’s great in the kitchen but when blowing or fusing glass, 2 different rates or amounts of expansion will lead to cracking. So, if you are a fuser and notice that the crack in your glass started along the line where 2 different pieces of glass meet, or if you work with hot glass and your piece shatters, you can pretty well expect that incompatibility was the culprit.