Pictured is a blue flame caused when Zinc vapour and oxygen reacts when heated to around 1000°C (in combination with other elements such as copper, silver, gold, hydrogen and nitrogen).
Zinc (Zn) decomposes into Zinc Oxide (ZnO: zinc vapor and oxygen) at around 1975 °C with a standard oxygen pressure. In a carbothermic reaction, heating with carbon converts the oxide into zinc vapor at a much lower temperature (around 950 °C). (Source: Wikipedia)
Here are some tips for when you are reusing your clean scrap or customer’s metal. Please comment if you have any tips you wish to share with us.
Be Clean and Tidy: If you know what’s in your scrap it will make it much simpler to troubleshoot any issues that might arise later.
The Periodic Table: You don’t have to have a degree in chemistry, but do try and learn about how metals behave. Remember that if they are close to each other on the table, they may behave similarly.
Precious Metals vs. Non-Precious Metals: Alloys usually contain non-precious metals which will affect their behaviour. High copper content such as reds and pinks can affect the crystal structure making the metals prone to cracking. As can nickel, or even silicon from casting scrap.
Quenching: Quenching in hot water (or metho) can help when dealing with alloys containing non precious metals. This helps by making the cooling rate of the different metals more consistent.
Fluxing/Gases: If your metal is questionable, an additional fluxing step is recommended to remove impurities. Likewise, if you see bubbles/flaring when melting it is worth cooling, then reheating the metal while stirring to try remove the gas. If the metal is still not acceptable after these steps we recommend that you refine and start again with fresh metal.
Oxidising: When melting/pouring, use of a cover flame will help avoid oxidisation. If this is not done it can result in burning off of metal (silver especially) but also in the hardening of the outside layer which can later cause issues especially if worked in to the metal.
Molds: This is purely a safety tip, but please make sure you heat your molds well before use.