On first read, Theophilus' treatise "On Divers Arts" is on the complex side, and he includes a lot of steps for things that sometimes border on mysticism. The most famous example of nonsense is his assertion that the urine of an old, fasting goat, or alternatively, a small redheaded boy, is most effective for quenching metal in hardening. His information on soldering isn't quite that outlandish, but it is rather complex.
Let's start by trying to demystify his instructions on soldering for silver. He starts with a recipe for solder that's a simple 2:1 silver to copper alloy that stays consistent through Cellini's treatise on goldsmithing, and isn't too far off from modern solder. So nothing strange there. Then he says to take argol (the sediment found in bottles wherein the best wine has been stored), wrap it in a cloth and burn it, blow away the ashes of the cloth, and then grind the burnt argol in a copper pot, together with salt and water to make a paste. He then instructs the goldsmith to spread this paste on his joints, and sprinkle shavings of the pre-made solder over it.
So "argol" is also called wine stone, or tartar, and is a crude form of potassium bitartrate. A relatively refined version of potassium bitartrate is found in baking: cream of tartar. Armed with this information, I have to wonder if ordinary cream of tartar, mixed with salt and water to make a paste, would act as a functional flux, when used with modern solder. So this is an experiment I intend to undertake in the very near future.
Secondly, let's look at his instructions on "Solder for Gold". Theophilus instructs the goldsmith to make lye using beechwood ash, and then to strain the lye water through the ash again, to thicken it. You boil this and reduce it by 1/3, mixing in with it some unspecified amount of soap and pig fat. Once cooled, he instructs us to strain it and store it in a specially described copper pot. Leaving that aside for the moment, he then tells us to take a flat, thin piece of copper, wet it, and then rub it with salt. Put the copper in a fire until it is red hot and then quench it in clean water. Continue this process, collecting the burnt copper oxides in the clean water until you have a quantity of such. Then pour off the water and allow the resulting powder to dry, and then grind it with an iron powder. Put the powder on coals and burn it again, and then grind it again. (This gets a bit repetitive.) Then add soap to it, mix it carefully, burn it again, and grind it again. At this point, pour the lye into the vessel with your copper powder and let it boil and reduce for "a long time". Once it's boiled and cooled again, pour it back into the copper vessel you were previously storing the lye in. This is its storage container. Theophilus tells us to add several pieces of copper to this container, and to stir them together with the liquid.
At no point in the instruction does he mention making a separate solder material. This resulting acidic soapy copper oxide is the only material he goes on to reference in soldering gold. The very helpful editor provides a note, at least in my Dover edition, explaining that this is a somewhat convoluted version of a process that was used for hundreds of years, and that Cellini describes a similar process, as well. The science is this: copper oxides are mixed with a paste flux, and applied to the metal joints. When heated, the copper converts to metallic copper, and, being in contact with the silver or gold, it immediately creates an alloy at the contact spot that's got a lower melting point. It creates a fusion of the materials without requiring the fine metal to get to its own melting point. Fusing at that point becomes less precise in nature, and does not result in the finely detailed filigree and granulation work that we see in extant examples. Fusing at the lower temperature created by the copper alloy preserves the delicate detail work.
Cellini's recipe for this is to use verdigris, mixed with sal ammoniac and borax. Which supports my assertion that the fundamental parts required to function in this way are copper oxides and flux. So in a slightly farther future experiment, I intend to acquire some copper oxide and sal ammoniac (I am not prepared to make my own lye at the moment), and see what I can accomplish!
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