More from the Oeconomia. Coler treats schmalz fairly briefly, in a way that is familiar to anyone in Germany:
Keeping pig or goose fat (schmaltz) so that it does not spoil in ten years
(marginalia: fat of pigs and geese, how to keep it long)
Boil the pig fat for an hour, but the goose fat only for half an hour, and add a little salt and onions (Zybollen oder Zwippel, caepas), thus you can keep it well over ten years.
Later in the book, when he speaks of keeping geese, he mentions another method:
How they slaughter and preserve geese in the country of Mecklenburg
You fatten a goose or several with oats and carrots cut small, for a week or four. Then you kill it (würget man sie ab, lit. strangle it), singe it all around and clean it nicely. Afterward, you cut the fat off their body entirely and skin them, keep the fat and use it throughout the entire winter. You chop it and cook it until it is done, lay it in a cask and weigh it down at the top. And you take some of it (greifft darzu) whenever you wish to have some of it. When a guest comes to visit, you warm up some of it. You preserve the fat with salt and coarsely ground oats and you chop it very finely, that way it stays nicely clean and keeps well throughout the summer.
p. 497 (646)
Animal fat was valued highly in Renaissance German cuisine, and we know that people used fat skimmed off soups and beef tallow in the kitchen if they had to. The most prized kind of fat was Schmalz, a term that derives from schmelzen (to melt). Today, Schmalz refers to the melted and clarified body fat of pigs or geese, but historic recipes often also subsume butter or butterfat (Butterschmalz) under it. Coler’s recipe is straightforward, but attractive, and it really is what you can still buy as Apfelgriebenschmalz in any German supermarket.
The preservation method for the goose fat is less clear. It sounds as though the fat is removed manually, chopped, cooked in water and then salted away, presumably together with the fat that rises to the surface during cooking. It is also possible to salt fat raw, but that does not seem to be intended here. Interestingly, the description suggests that this recipe served the needs of a small household. It may have been the kind of small luxury a smallholder or cottager could offer an honoured guest.