In one word:
DON'T.
This is a real safety issue. I have had the bottom of a flask falling out from dissolving lye in it while holding it in a bucket of fast moving water. Luckily, the bucket served to contain the hot concentrated lye spill (and of course I was wearing safety glasses and gloves). Since then, I dissolve lye in a tall slender stainless steel pan. If you must use glass, at least use a glass coffee or tea pot. These are made from thin borosilicate glass, the same glass that laboratory beakers are made from. Some pots even have a "Schott Duran" logo.
Always use a secondary spill container, eg. a bucket. Not only while dissolving lye, but also to hold the extraction container. Put the extraction container in a sturdy plastic bag while you are shaking the layers. If an accident happens, it will be largely contained instead of splashing over the tapestry and your clothes (or even you).
Besides the safety issue, it is a bad idea anyway to dissolve the lye directly into the plant sludge. The dissolving lye creates local pockets of superheated and very strong lye solution. These are incredibly corrosive and their effects on the plants and molecules has a negative effect on the quality of your extract. My experience is that using pre-made lye solutions gives a much cleaner end product.
cyb said:
Glass can handle the temp if it's thick enough...
AFAIK, actually glass is better able to withstand temperature gradients if it is thin. As an illustration, the only thick laboratory glassware are vacuum filtration flasks and vacuum dessicators, and those are not intended for temperature gradients, only pressure gradients.
