So-called "wine diamonds," (harmless)
Potassium bitartrate deposits which can accumulate
in bottles and barrels of wine
Neri notes that tartar also went by the name of "gruma" to which we can add the synonyms greppola and argol. He warns readers several times to "leave behind the [dried] powder of the [raw] tartar, which is no good" and that "you should have tartar, from the dregs of red wine, which is better than white wine."  Nevertheless, he does specifically use white wine tartar in his recipes for rosichiero, a transparent dark red enamel.  In his recipe for producing tartar salt  he directs the reader to obtain the raw material from emptied wine barrels, but elsewhere in the book he seems to prefer to use the large crystals of tartar that have “vitrified naturally in bottles of wine.”  Chemically, tartar is a potassium compound formed through a reaction with tartaric acid, a major constituent of grape juice. 
The effect of tartar in improving the appearance of glass can be readily explained. Most of the glass that was made in Italy in the seventeenth century used sodium-based additives to lower the melting temperature of finely ground quartz powder; these formulations are known as soda-based glasses. Using potassium compounds can have the same effect and these 'potash' based glasses were predominantly produced in northern Europe where trees and plants rich in potassium were used in glass. Potassium is a heavier element and it produces a denser, more refractive glass, giving it more sparkle, although not as much as lead imparts to crystal. Unfortunately, potassium also makes the glass harder to work for the artisan. Potash glass stiffens more quickly as it cools, whereas soda glass remains workable for a longer time before requiring reheating in the furnace. Many of Neri's recipes blend the two additives, which we can imagine gives some of the advantages of both.
Neri also used tartar to modify color in glass. The effects of tartar are exemplified in a number of passages throughout the book. He uses it as the sole pigment in his recipe for pearl colored glass, but he warns, "Once obtained, you must work the color quickly, because it will dissipate."  Conversely, he also uses tartar to produce a black colored enamel, combining it with manganese [oxide], which by itself imparts a magenta color. 
The first step in making Neri's purified tartar salt is to obtain the raw "gruma, from barrels of red wine in which it forms large lumps." Next, he gently roasts it in terracotta pots "until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good." Now, he boils it in water for two hours, evaporating off three-quarters of the liquid. After filtering, he lets the remaining liquid "lye" cool in pans, allowing any sediment to settle to the bottom. He gently pours off (decants) the liquid which is further processed on the stove, this time in glass containers. The result, after full evaporation over a slow fire is a “white salt” left in the vessel. He dissolves this in hot water, filters it again and allows more sediment to settle out for a period of two days. Again, the liquid is decanted and evaporated in a glass container. The filtering and evaporation process is repeated four times, resulting in a product that is "whiter than snow."
Neri's final remarks for this chapter are as follows:
“When mixed with sifted polverino, or rocchetta, with its doses of tarso or sand, this salt will make a frit that in crucibles will produce the most beautiful cristallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it, good fine cristallino can be made, nevertheless with it, it will be the absolute most beautiful.” 
 Neri 1612.
 Ibid, ch 116, 117.
 Ibid, ch 113. Note that in glassmaking, the term 'flux' has a different meaning than in metallurgy.
 Ibid, ch 41.
 Ibid, ch 125.
 Ibid, ch 11.
 Ibid. ch 46.
 Pure tartar takes the form potassium bitartrate KHC4H4O6.
 Neri 1612, ch 60.
 Ibid, ch 102.
 Ibid, ch 11. Polverino and rocchetta are thought to be forms of dried Salsola Kali plants. Tarso is Neri's term for white quartz river stones. Cristallino was a Venetian style glass that in quality fell between common glass and the premium cristallo, for which Murano became famous.
* This post first appeared here on 5 Sept. 2014.