# How to increase the temperature of my double boiler?

I have a double boiler that I wish to use to prepare a batch of fudge. The recipe I am using calls for heating to 240°F (=116°C), but a double boiler is limited to the boiling point of the working fluid used, so plain water, boiling at only 100°C, won't work.

I know that one can increase the boiling point of a liquid by dissolving material in it, governed by ∆T = Kb m, where Kb is the boiling point elevation coefficient and m is the molal concentration of solute.

Using this, to get the boiling point of water to 116°C requires a ∆T=16 K, and Kb = 0.512 K / m, so that would require a 31.25 molal solution. Using NaCl, that would mean about 1.8 kg of salt per kg of water, which is around 5 times as much salt as will even dissolve at all.

What can I do to increase the temperature of my double boiler?

• To move it that far, you'd have to use some other liquid, as a fuly saturated brine will only get you to 108°C / 226°F. Unfortunately, you'd have to find something that was food safe and didn't cause irritation in gaseous form. Another option might be to try using the oven, rather than the stovetop. (to at least get it close, then finish on the stovetop, watching it like a hawk?)
– Joe
May 10 '15 at 17:44
• Why do you specifically want to use a double boiler for this application given the inherent temperature limitation? May 10 '15 at 20:33
• Sugar solutions (sucrose/water) will get you there. Soft ball stage runs about 115 °C: craftybaking.com/howto/candy-sugar-syrup-temperature-chart Can't find a decent table of BP°C vs percent sugar today, or I'd make this an answer. May 10 '15 at 20:57
• @WayfaringStranger I wouldn't try to make a sugar-solution double boiler. The problem is that a suger solution's temperature is not limited, and once they start supersaturating, they spend very short time at each temperature, frequently even less than the reading delay of a sugar thermometer. You can certainly take a sugar solution to 115 C, but you cannot hold it there for a prolonged time without some complex apparatus (which would have been capable of doing the double boiler duty in the first place).
– rumtscho
May 11 '15 at 8:48
• @rumtscho Agreed, now that I think about it. If you really want some oddball temperature, you're better off with a precisely thermostatted heating plate. May 11 '15 at 12:46