The green flame
The color of the flame coming from the melting furnace should be slightly green. The green color is emitted by the burning copper vapor which escapes from the melt bath. Every metal burns with a specific color, copper green and is the most dominant metal in the alloy. The oxygen is supplied by the air which is not used by burning the fuel. This excess air containing oxygen is needed to oxidize the top of the melt bath. When oxygen is available on the melt bath a film of copper oxide develops in a few seconds. This film seals the melt bath from the furnace atmosphere. The furnace atmosphere contains besides the normal air components, the products of the burned fuel, coal dioxide and water. If melt bath is not sealed the water vapor migrates in the melt bath, the hotter the faster. When this bronze is poured and solidifies, the bronze cannot contain the quantity of water anymore and bubbles water vapor develops in the solidifying bronze. Similar with air bubbles in ice. The bubbles make holes of a few mm in the casting, too in the surface.
Flame with an excess of air
The good flame with a small amount of excess air
Flame with not enough air can lead to gas porosity in the casting
Small holes in a silicon bronze casting caused bubbles which develop in the solidifying metal