Silver salts are used to create the yellows, ochres and browns. Silver reacts with hot glass. It turns blue glass green; clear glass yellows, ochres and browns; and red glass into brown.
A salt is a molecule of metal, chemically bonded to other molecules - for example chlorine or a nitrate. Table salt is sodium chloride.
When these salts come into contact with heat, they dissociate. The bit of metal reacts with the hot glass, and the chlorine becomes a gas - this makes bubbles.
Nitrate seems to create bubbles too, but I don't know if that becomes a gas - it might just provide seeds for a bubble to form around.
The fact that Mdina actually created their own colours was a big selling point initially. They did it less and less as time went by. Said took over and colours became commercially more available and they, in time, managed to raise the quality of materials used.
Early on, they had trouble sourcing raw cullet. This is why the clear glass used in some early stuff is of a very poor colour - they ended up using recycled milk bottles.
There was also the matter of the silver salts being extremely toxic, especially when heated to the point they dissociate and turn into gases!
The silver metal turns into a gas too. It was getting some of that to escape that creates the silver trails deposited on the outside of some pieces.
Initially an accident, it was a much desired technique for one of the makers there to learn to accomplish. Not everybody could.
I have often wondered if breathing in these chemicals contributed to Mr Harris' emphysema. :'(
Tom, we cross-posted a short while ago, check if you've seen my post, and we're cross-posting again now, but I know about this one.
You are correct. Don't go reading anything into the small differences.
The whole point of it being studio glass is that each piece IS unique.