I like this experiment because not only do you get to explore/demonstrate the effects of thermal shock*, but you also get some lovely glasses at the end. Also, you’re recycling and the earth will love ya for it.
I would recommend following the materials list closely. Originally I did not and tried to use acrylic yarn instead of cotton. This was a mistake because acrylic yarn burns instead of melts and I wasted quite a few bottles and a lot of patience before admitting defeat to the acrylic gods. Also, don’t be worried if the first couple of bottles you try don’t break evenly – it’s like the first pancake principle – you’ll get the knack of it eventually.
Step 1. Decide how tall you’d like the glass to be and wind the yarn around that point of the bottle – don’t make it too tight.
Step 2. Soak the yarn in acetone for about a minute.
Step 3. Put the yarn back on the bottle and wipe off any extra acetone.
Step 4. Light the yarn on fire. Hold the bottle by the neck and as it’s burning slowly rotate it to ensure that the ring is heated evenly. Keep turning until the flame goes out.
Step 5. Put the bottle in the bucket of ice you prepared earlier. If all has gone to plan you will hear a crack and the bottle will have snapped around wear the yarn was tied.
Step 6. Sand the edges of the bottle until it’s velvety smooth and won’t cut your face when you try to drink.
*Glass is notoriously bad for cracking under pressure. But what pressure you might ask? The pressure of thermal shock. Thermal shock describes the way in which materials are damaged from a rapid change in temperature. When glass is heated it expands and when cooled it contracts. As glass is also a bad conductor of thermal energy, a thermal gradient occurs because its molecules expand and contract unevenly. This unevenness puts stress on the molecules and if this stress is too great the glass will crack and/or explode. In this experiment, we are concentrating the expansion and contraction in one place (where the yarn is being burnt) this results in a clean break of the glass when it is dropped in the ice and therefore a nice glass for us.