So after the brass alloying experiments, I've been doing some reading about foundry coagulants and what they're made of. This article http://www.schundler.com/foundry.htm about perlite mentions it's service temperature limit is 1100 °C and that it can be used to coagulate iron and steel slag into a lump when it melts at 1300 °C. Another article mentions perlite for brass, bronze and iron use: http://blog.hillandgriffith.com/foundry-blog/molten-metal-slag-coagulant With a bit more searching I found a perlite processor less than an hours drive away and rang them this morning: "Hi I'm looking for some unexpanded perlite?." the guy on the other end replied "Yeah we have some of that, we sell it to foundries all over the country as a coagulant." "Oh?, that sounds like the stuff I'm after". So by lunchtime I had a 20 litre pail of kiln dried but not expanded perlite granules. When it's heated with a propane blowtorch it puffs up a bit and gets soft but not really sticky and sort of clumps together the grey slag skimmed from the brass melting session but it doesn't quite form a solid lump. Now I know it's form of volcanic glass so I tried a few additives like lime (glass flux), sodium silicate before hitting on the combination of perlite and sodium carbonate (also glass flux) 50-50 where I made a damp slurry and dried it out with the torch....Bingo!!!. The melting point is lowered significantly where the propane torch can melt it into a pale blue-green-white glass and form a sticky barely liquid mass. So the sodium carbonate is fluxing the perlite enough to lower the melting point significantly. I'll have to test it to fully prove the recipe but results so far look good with a lower melting point than normal run of the mill glass chips or perlite alone, making it handy for brass and bronze use as well as iron service. Edit: The sodium carbonate appears to melt and flux the perlite to form glass, heating perlite or sodium carbonate on their own doesn't give a result as good. I'd expect on top of molten brass the heating and mixing of the two will be better and give good results. Kiln dried, unexpanded perlite for iron foundry use: Perlite/sodium carbonate glass after propane torch heating: Epanded perlite is the white lumps all other granules/lumps are unexpanded perlite.
I'm running at lower settings since fitting that diffuser plate and there's a lot of clearance between the lid and the crucible so the iron slag tornadoes are a thing of the past. From what I've seen with using coagulant on bronze, you sprinkle it on top, let it melt maybe give it a stir and everything clumps to the skimmer and you pull a not quite solid lump out with the viscosity of molten glass. So it's an aid to skimming before fluxing the bronze. The commercial bronze stuff has other additives, but the pure stuff is used on iron.
Today I scored a sample of commercially sold coagulant intended for copper based alloys and heated with a sample of the unexpanded perlite for a side by side comparison. They both behaved identically and expanded and became slightly sticky and the grains stuck together. The commercial sample is obviously some kind of crushed mineral rock and is a white colour compared to the dried unexpanded perlite's "dirt" colour with the undried raw mineral being a very dark green colour before drying. Perlite seems to have a wide variety of colours before turning white and puffing up when heated. The larger sample on the larger firebrick is the experimental local perlite and the smaller sample on the small firebrick chunk is the commercial product intended for copper alloys. After seeing the two side by side, I fully expect to use the unexpanded perlite as-is on brass and bronze as well as iron, possibly with soda ash/sodium carbonate to lower the melting point on aluminium. Local unexpanded perlite is on the left, commercial copper alloy coagulant on the right hand side.....seems identical. Commercial copper alloy coagulant: Locally sourced kiln dried unexpanded foundry perlite:
