Since I don't know this product, I can only answer with (my) common sense:
Air should be a much better insulator than glass, so even if there's no (good) vacuum, the insulation should work pretty well.
One thing to keep in mind is what is mentioned on the Wikipedia article on vacuum flasks
Heat transfer by thermal radiation may be minimized by silvering flask
surfaces
and observing that a normal vacuum bottle isn't translucent. That means that I would expect some loss due to radiation, since the glass isn't covered in any way.
It looks like you are trying to make sugar fondant. I often make a batch to use as seed crystals in my holiday fudge preparation.
Sugar Fondant
Sugar fondant is a crystalline sugar confection where the crystals are microscopic and suspended in a saturated solution of sugar. Its texture is very short, and the mouth-feel is creamy.
Creating sugar fondant is relatively easy. In brief, boil a syrup to softball stage and cool it undisturbed until around 50C followed by rapid agitation until the fondant is too difficult to work.
Now for some specifics.
Multiphase Solutions
One of the interesting features of boiling syrups is the temperature is intrinsically linked to the composition of the syrup. Unlike how boiling water transitions to steam at a constant temperature of 100C, syrups boil at a range of 110C to well over 200C. As water evaporates from the boiling syrup, the composition changes to contain a higher concentration of sugar and the boiling temperature rises.
Be warned. You are boiling a super-saturated solution. Any crystallized sugar introduced to the solution will not dissolve and it will seed crystallization during the cooling stages.
- Avoid stirring once the sugar has fully dissolved prior to boiling. Use a brush moistened with hot water to wipe away any crystallized sugar on the side of the pot during cooking.
- If you are adding any glucose to the syrup, add it after the syrup has come to boil to ensure that the other sugars have fully dissolved.
Candy Stages
The desired properties of a candy are principally derived from the candy stage to which you cook your syrup. The sugars remain mostly unchanged chemically (though disaccharides may break down into glucose and fructose) regardless of the candy stage or temperature.
When making sugar fondant, you are aiming for the softball stage which is 110C to 120C. You can take a dollop of hot syrup and drop it into cold water to check the stage in absence of a good thermometer or if altitude / humidity are affecting your candy. So long as you do not scorch the sugar, you can add water and lower the temperature to restore the syrup to the desired candy stage.
Crystallization
The most important part of a sugar fondant is the formation of microscopic crystals. The syrup must cool to 50C before agitation to create the desired crystal size and distribution.
One difficulty that I have found is that I cannot let the syrup cool within the bowl of my stand mixer - the syrup cools unevenly which typically induces crystal growth. This is especially difficult for fondant, as it must be worked for a significant amount of time to crystallize.
Depending on your desired purpose for the fondant, you may let the syrup cool to a lower temperature or agitate less to keep the texture longer or more pliable. If you agitate at a higher temperature, the syrup will form crystals that grow tremendously during agitation and make the fondant grainy or crunchy.
But Why Is It Crunchy?
Chances are, you have undissolved sugar in the syrup.
- Be sure all of the sugar dissolves before you add any glucose to your syrup.
- Don't stir once it begins to boil.
Your syrup might be at a higher candy stage. If you somehow managed to reach soft crack or hard crack, this might prevent the syrup from forming any crystals and resulting in an amorphous sugar glass.
Best Answer
Thickness is not an indicator of heat-proof glass, just think about laboratory glassware, which is sometimes quite thin. It’s rather the other way round, thermal conductivity is low for glass which can increase inner tension when heated quickly. Thicker glass will be more robust against mechanical force, simply because it’s thicker.
If you want to be sure, get a bowl that is labeled as “heat-proof” or similar (terms may vary). It’s still a good idea to avoid sudden temperature swings, just to be sure although some manufacturers claim that their products can withstand temperature swings of 150K. It shouldn’t be an issue with melting chocolate, because you should place the bowl over (not “in” or “touching”) barely simmering water anyway and not have the water in the lower pot at a full rolling boil. There’s a good chance that even non-ovenproof bowls will stand up to this, but as you experienced, it can go wrong.
Another, often overlooked factor is the tension within the heated glass, especially if the temperature distribution is uneven (again: don’t boil the water at full speed). The glass will be more sensitive to shocks, so don’t bang the bowl with a metal spoon when stirring or getting leftovers off the spoon and don’t set it down hard on the table or counter.