The fourth essential is melting order

When I commenced to formulate these alloys, having failed to find published instruction, I was neither assisted nor restricted by prior knowledge. It was this freedom that allowed me to experiment with silicon.

Results were achieved purely by trial and error and observation. Initially I just melted the metals together without any order. Then observing the way the metals melted with their quite different melting points and vaporising points, I soon deduced there should be a definite order for melting the metals.

Due to the very low boiling point of zinc, melting order is particularly important for any zinc-containing alloy. The order I have established is (after preglazing a crucible with boric acid) place the zinc with a pinch of boric acid in the bottom of this preheated crucible and gently melt with a reducing flame. The copper, with the silicon copper, is now placed on top of the zinc. With each addition of metal add another pinch of boric acid. All the time maintain a reducing flame on the melting metals.

By melting in this order, the molten zinc is drawn to and begins to alloy with the copper as it is heated. The silicon copper begins to alloy with the copper, further lowering the melting point of the alloy as it is forming, bringing it closer to the volatilisation point of the zinc. At this point a small amount of zinc oxide may be formed but as the silicon begins to alloy with the other constituents, it scavenges the oxides.

The melting can continue without fear of more zinc oxide being formed, provided the temperature is not allowed to rise too high and a reducing flame is maintained. The result is an oxide-free alloy. Although zinc oxide fumes are not regarded as toxic, inhalation can cause flu-like symptoms and a temporary illness. Avoid inhaling zinc oxide fumes.

What one must bear in mind are not only the melting point of zinc, 419°C, but also the boiling point of 907°C, the boiling point being below the melting point of any of the other constituents (Figure 10). At the point of volatilisation of zinc, any oxygen present will combine with the zinc and produce white zinc oxide fumes. I found zinc oxide to be a major cause of porosity problems. If not kept to a minimum, zinc oxide remains in the metal and leaves random porosity in any part of the casting.

It is preferable that the alloy be melted without volatilising the zinc. Gas torch melting makes this a difficult, but not impossible task, depending on temperature/flame control and the order of the addition of the metals.

It is particularly important to control the formation of zinc oxide as once formed it is unlikely, under these conditions to reconvert to metallic zinc. This was proven to me quite graphically about 6 or 8 months after I had started using this system. Finding I no longer had the time to make the amount of master alloy I needed, I asked my refiner to make 50 kg’s of master alloy. Immediately random porosity was evident in the castings, so I went to see how the master alloy was being made. I arrived in time to see the silicon copper in a molten state and the furnace-man throwing the zinc on top. This of course filled the furnace room with zinc oxide fumes and the resultant silicon brass with zinc oxide inclusion. I reiterated my procedure for alloying zinc-containing alloys. When this alloying procedure was adopted, I had no further problems with zinc oxide inclusion.

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