Topic: Large Mdina knopped chalice

[chriscooper]

It is amethyst John you are quite correct see it better from the inside the fuming (is that the right word) with no light through looks like oil on water.

[chopin-liszt]

It's not fuming. That's a deliberate technique of a spray applied to the surface of a piece at the very end of production.

For the effect found on Mdina, Silver Chloride salts were used to add colours to the glass.

When it was all hot and inside the molten metal, the silver and the chlorine ions become dissociated, allowing the silver to react with the glass (the chlorine just escaped as a gas or might contribute to creating bubbles), and if the contact point between the moil and the metal got broken accidentally during the working, silver metal escaped from inside as a gas and became deposited on the outside of the piece.

This was discovered by accident, but would become something they incorporated deliberately, in order to achieve the lovely silvery bluey-yellowy effects on the surface.

Breathing it all in very probably contributed to the emphysema that killed Michael Harris.