In this video I cut jars and bottles into rings and cylinders, by using controlled thermal stress a bottle can be cut with a beatify clean/smooth finish that requires the very minimum of finishing. The approximate coefficient of linear thermal expansion of glass bottles is around 9 x 10-6 cm/cm oC. It gets more complicated if you consider the thermal expansion of 'volume', but suffice to say, we can induce a critical temperature difference in the glass that will help us. We want to use 'thermal stress' rather than 'catastrophic thermal shock' to split the glass along a score line made with a glass cutter. To achieve this we heat the glass up and let the heat soak right into the body of the glass and then quickly cool the glass, preferably inside and out. The change in temperature causes a minute, but very real, change in the physical size of the glass, now if we can bring about a change in temperature very quickly the glass on the outer surfaces will contract faster than the glass under the surface creating stress within the body of the glass. Glass is very strong in compression but weaker in tension so effectively the glass pulls itself apart usually leaving a beautifully clear/regular fracture line far smother than you can achieve by using mechanical shock (tapping the cut line) or by nibbling (breaking with piers) Remember if there are any other scratches or chips in the glass they can (and probably will) cause the glass to brake where you don't want it to. From my trials I've found that using simmering/boiling water to heat the glass followed by a plunge into cool tap water (at around 8 oC ~ 46 oF) is enough to bring about the desired results. Frequently I need to repeat the heating and cooling cycle several times. Remember to let the heat soak into the thickness of the glass, that takes time. Not all glass behaves the same and some bottles need to be heated and cooled several times and in worst case scenarios you may need iced water to create sufficient stress to part the glass. Typically clear glass needs a temperature difference of about 70 oC (130 oF) But surprisingly, some coloured glass needs several cycles from boiling water to iced water to split it. If the glass doesn't separate completely with in the first couple of heat cycles try to resist pulling the pieces apart manually as the best results come from thermal fatigue leaving you very clean surfaces that won't need much attention to finish them off. Obviously wear safety goggles and any other appropriate safety gear. Be prepared for the glass to shatter unexpectedly, I haven't had it happen but I remember as a child, shattering a glass milk bottle when I tried to fill it with boiling water, my first brush with 'catastrophic thermal shock' Thanks for watching Kind Regards ... Andy GWØJXM

• published: 30 Nov 2013
• views: 429