|The chapel of Santa Cristina, overlooking|
the valley near Monte di Voragno, in
Piedmont, Italy where manganese ore was mined
for the Venetian glass industry.
The translation from Italian in that posting was by yours truly, so the first order of business must be to revisit the original text, which these days, is conveniently accessible online here. On page 104 (recipe # 120) of Neri’s original 1612 edition of L’Arte Vetraria, (lines 11-12) we find: “…di questa medicina si da venti per oncia di cristallo ò vetro...” [of this medicine will be twenty per ounce of cristallo or glass].  So it seems my high-school Italian is not to blame after all.
The second edition of Neri’s book was printed in 1661, almost fifty years after the glassmaker’s death.  The editor claims the correction of a number of errors, but these turn out to be mainly changes in grammatical convention; our passage is identical to the original. Likewise for Christopher Merrett’s 1662 English translation: “…there are used of this medicine 20 ounces, to one of crystall or glass…” and he makes no further comments in his observations section. So it appears for the twenty odd other editions that published over the next two centuries in Latin, German, French and Spanish.
Nevertheless, something is definitely amiss, since in his other recipes Neri always specifies the amount of glass in pounds; the only times he uses ounces are in prescriptions for artificial gems when quite small amounts of material are being prepared. Perhaps we can find a clue elsewhere in the book. In chapter 13 he describes preparing the raw manganese ore, mined in the Piedmont region of Italy. He says, “In Venice, you can always find it in abundance, since on Murano they do not use any other manganese.” For his garnet color (#47) he uses one pound of manganese to one hundred pounds of glass. For a garnet color in lead glass the ratio is 20 pounds of cristallo, 16 pounds of lead oxide, and three ounces of manganese.
Manganese oxide played and still plays a versatile role in glassmaking. In relatively small quantities it performs the role of a color neutralizer, removing the slight greenish tint introduced by iron contamination in glass. On the other extreme, it was used by Neri in conjunction with potassium carbonates and cobalt oxide to make black glass, but never in quantities more than a few percent by weight. Between the two extremes, manganese was used to make violet, wine, garnet, and amethyst colored glass.
Judging from these other recipes and Neri’s failure to explicitly remark in this one about the outlandish ratio, we are forced to conclude that the twenty to one ratio of color to glass is an error that slipped through, apparently unnoticed or at least not commented upon until this week. I have a fondness to gravitate to the simplest explanation, which in this case is an inadvertent reversal of the amounts; in other words the ratio should be one part pigment to twenty parts glass. Unfortunately this does not speak to why he is quoting amounts in ounces when pounds are his norm.
As unsatisfying as it is, a well reasoned answer to this riddle does not present itself. Perhaps Neri left a clue in another of his writings. Sometimes, when we follow the facts, historical research forces us to take the most courageous stand of all: we simply do not know.
 Neri 1612.
 Neri 1661.
 Neri 1662.