As quoted in Miracle Mongers And Their Methods By Houdini

Chambers' Book of Days, 1863, Vol. II, page 278:

The performer taking an iron spoon, holds it up to the spectators, to show that it is empty; then, dipping it into a pot containing 'melted lead', he again shows it to the spectators full of the molten metal; then, after putting the spoon in his mouth, he once more shows it to be empty; and after compressing his lips, with a look expressive of pain, he, in a few moments, ejects from his mouth a piece of lead impressed with the exact form of his teeth.

Ask a spectator what he saw, and he will say that the performer took a spoonful of molten lead, placed it in his mouth, and soon afterwards showed it in a solid state, bearing the exact form and impression of his teeth. If deception be insinuated, the spectator will say. "No! Having the evidence of my senses, I cannot be deceived; if it had been a matter of opinion I might, but seeing, you know, is believing."

Now the piece of lead, cast from a plaster mould of the performer's teeth, has probably officiated in a thousand previous performances, and is placed in the mouth between the gum and the cheek, just before the trick commences. The spoon is made with a hollow handle containing quicksilver, which, by a simple motion, can be let run into the bowl, or back again into the handle at will.

The spoon is first shown with the quicksilver concealed in the handle, the bowl is then dipped just within the rim of the pot containing the molten lead, but not into the lead itself, and, at the same instant the quicksilver is allowed to run into the bowl. The spoon is then shown with the quicksilver (which the audience takes to be the melted lead) in the bowl, and when placed in the mouth, the quicksilver is again allowed to run into the handle.

The performer, in fact, takes a spoonful of nothing, and soon after exhibits the lead bearing the impression of the teeth.

Molten lead, for fire-eating purposes, is made as follows:

     Bismuth . . . . . . . . . . . . . . . 5 oz.
     Lead. . . . . . . . . . . . . . . . . 3 oz.
     Block tin . . . . . . . . . . . . . . 2 oz.

Melt these together. When the metal has cooled, a piece the size of a silver quarter can be melted and taken into the mouth and held there until it hardens. This alloy will melt in boiling water. Robert-Houdin calls it Arcet's metal, but I cannot find the name elsewhere.