One of the most important factors to achieve success in working colored borosilicate is flame setting. In general, the majority of the colored borosilicate palette is affected by reduction. It is critical to know and understand the flame chemistry to be able to control the color. When looking at flame chemistry there are three categories of flames: reducing, neutral, and oxidizing. There is much subjectivity in defining how oxidizing or reducing a flame is, but it can be judged relatively well by flame color, candle length and the sound.
A reducing flame is one in which the gas is not being fully combusted. This means there is not a sufficient amount of oxygen present to burn all the gas. Its long wispy candles and soft bushy character easily identify this type of flame.
A neutral flame is one in which there is an equimolar ratio of propane to oxygen. This means that there is enough oxygen to fully combust all the gas. A neutral flame is the hottest flame that a torch can put out. In identifying a neutral flame look for the sharpening of the candles and for a bright blue glow. This indicates that the flame is neutral.
An oxidizing flame is any flame that has an excess of oxygen present. A hissing noise, sharp candles, and a paler blue color easily identify the flame. This flame is cooler than a neutral flame because there is an excess of oxygen flowing though the combusting gasses.
Learning and identifying these flames is critical in having good control over the colored borosilicate palette. A good litmus test in learning to dial in these flames is by working a piece of NS-27 Green Exotic. Start by dialing an oxidizing flame in, then place the rod in the flame and gather a small glob of glass. Take it out of the flame and inspect the color. If the rod is still black this indicates that the flame is oxidizing enough. Second, dial in a soft reducing flame and work the rod. A metallic sheen should rapidly develop. If this metal deposit occurs when the oxidizing flame was dialed in, this indicates that there was not a sufficient amount of oxygen present. For those just starting out with borosilicate, this is an excellent exercise to practice and get comfortable with the flame.
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