Well, if you know (as we're sure you do) that tartar is the white crystals that precipitate out of wine as it is aged, and that deposits of tartar are scraped out of wine barrels, purified and then ground to a powder, the German gereinigter Weinstein (purified wine stone, or crystal) will make perfect sense. It should also get you what you need in the market - although, frankly, we have scoured the shelves of German stores without finding any cream of tartar. It is available in most pharmacies.