Manganese from Piedmont

For Antonio Neri, contaminants, especially metallic contaminants were the bane of producing crystal clear glass. Great care needed to be taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will tint glass green and for Renaissance era glassmakers, nemesis iron was everywhere. It is common in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

The antidote to iron is manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions "… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark."  

Removing the green tint of iron with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if neutral in color. In terms of what light does, when it passes through glass tinted by iron, green light is unaffected, while red and violet light is dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.