How did pyrolusite help Venetian glassmakers reduce unwanted color in glass?

Venetian glassmakers used pyrolusite—a manganese mineral also known as glassmaker’s soap—to eliminate unwanted colors in their glass.1

In the production of primitive glass, impurities such as iron salts often resulted in a natural, undesirable smoky or greenish tint. By adding pyrolusite to the glass melt, the glassmakers were able to neutralize these accidental colorizers. While this process naturally resulted in a gray tint that slightly reduced the overall transparency of the material, the trade-off was considered beneficial; as long as the amounts of the original colorizer and the pyrolusite were kept small and the glass was not too thick, the loss of transparency was far less noticeable than the unwanted color would have been. This technique was essential in the development of cristallo, the clear, colorless glass that became a hallmark of Venetian craftsmanship.2314