The tin side of soda lime glass is harder than the air side. The tin side has diffused tin atoms that make it hard. It is harder than Pyrex/Borofloat33/Supremacy. This is confirmed in Schott PDFs, glass scoring sites, and was told to me on a glass fusing site.
Another PDF says Borofloat takes twice as long to wear down as soda lime does, and requires almost twice as much minimum force for the first scratch. It is not stiffer. The scratch resistance comes from its tensile strength, which helps if you drop the mirror but not with flex in a cell. I'm guessing the tin side of soda lime is 3x the hassle of the air side.
People scoring glass look for a non-bevelled edge and look for which side started with a score and which side a snap. They assume that the cut was done at the glass place by someone who knew not to where down their wheel on the tin side. But most glass is beveled or sawed. So the other way to tell is with a $40 short wave UV light, not a black light. Darken the room, shine the 254 nm into the glass, and see which side has glowing tin clouds. Homesciencetools has one. Do not look into this light! I would wear polycarbonate safety glasses when doing this, since it naturally blocks UV. Test your googled once by placing them between the light and the glass. If you still see tin clouds, the glasses don't block UV.
I'd you hog your curves, the difference between the sides matters a bit, but not after you break through to the soft inside. It matters far more to those who slump or treepan:
Slumping, you want the soft side to be concave, and the tin side convex. That way, you can start fine grinding and not have to work 3x as hard. It is harder than Borofloat33 remember.
Treepaning, the most difficult part is getting the hole started. If you don't want your blade dancing around a long time before digging in, then put the tin side on the other side and start on the air side.
Treepaning surfaces should be wet enough to avoid glass dust getting in your lungs, or heating up the glass and blades. I recommend at least 1/8" of sitting water if you are cutting fast. With glass, blade speed should be slow. Fast metal can fling grit, and metal flanges can flare as they go in. Always let the abrassive do the work, while pressure merely follows it.
Comments or other tid bits?
Also, if the glass has any scratches, those could precipitate cracks, by greatly reducing how much stretch is needed to start one in the kiln. Used table glass should be polished like brand new if you want faster heating and cooling.
However, it is the cooling that would cause a surface scratch to crack. If the glass is heated hot enough, they might disappear. But at the tack fixing temp, it can also stick to the kiln if kin wash is not used. A disposable surface might be good for initial tests.
Edited by stargazer193857, 06 December 2019 - 11:55 AM.