Learning to Edit
Double Boiler
In most cases, a normal saucepan is sufficient and a double boiler is unnecessary if the cook is willing to stir constantly over very low heat. Some of the recipes here (Seven Minute Frosting, for example) require an older style of double boiler. These are extremely difficult to find, so we hope that this page contains enough information to enable you to figure out whether the double boiler you've found is adequate. If you cannot find a double boiler, it is theoretically possible to place a bowl over a pan of boiling water to achieve the same effect, but this can be very awkward to work with unless you practice a lot and have the right combination of pan and bowl sizes.
In the 1900s, Corning Pyrex double boilers were the standard. They required little metal standoffs so that the glass didn't break, but they provided low even heat around the bottom and sides of the pan, even when the burner heated unevenly. They were fairly small in diameter (less than 7 inches), but tall.
This is the gold standard of double boilers. The base outer pan is very deep to allow the water to simmer under the insert. Note how the base bulges to allow the steam to circulate around the insert:
The insert pan is also deep (8.5 cm), so that the heat surrounds the contents:
The metal band around the top attaches the handle, and the bulge at the top of the pan allows it to rest on the lip of the base. This pan can be used to make a double batch of Seven Minute Frosting, but the frosting will come up or over the metal band region of the insert.
In the 1990s, a company made metal double boilers of the appropriate shape.
These are a little smaller than the pyrex double boilers, but can just hold a double batch of Seven Minute Frosting. These too have disappeared from the market.
Modern Double Boilers
There are several types of modern double boilers.
High end ceramic deep insert double boilers (marketed for melting chocolate and cheese) are typically too small to fit the beaters into and therefore can't be used to make Seven Minute Frosting.
Metal or pyrex shallow insert double boilers (usually included with sets of pots for cooking sauces at low temperatures - again, stirring constantly in a normal saucepan over the lowest temperature setting is usually just fine) are primarily useful if you have a gas stove where even the small burners run hot and you need to cook something over very low heat and don't want it to get lumpy. They are characterized by an insert that sticks up from the base and has a shallow overlap with it. You can make Seven Minute Frosting with them, but it won't get fluffy.
Universal double boiler inserts seem like a good idea, but if you try to make Seven Minute Frosting with a conical one, the beaters won't be able to get to the frosting and it won't work. The Neeshow double boiler universal insert looks like it might work (but we haven't tried it, so no promises).
Your best bet for a proper deep insert double boiler is to search for "pyrex flameware" on eBay or Etsy. If Replacements.com still exists, you could try http://www.replacements.com/webquote/pyrfla.htm
Images provided under CC-BY 4.0. Please attribute to Redfield Recipes.