Never heard of 15 phosphor-copper?
Neither had I.
My bronze castings had been coming out of the investment mould looking like this. I had been expecting to see a wonderful shiny golden bronze colour, and was a bit disappointed in the irregular black coating. But the casting itself was complete, with no voids, and having fine details like fingers, and 3D printing marks showing up clearly. And after laboriously attacking the black coating with Dremel wire brushes, files, sulphuric acid (ineffective), a lot of the black was removed.
This baby T Rex has found a home on my model dredger engine. The variegated skin colouring is reasonably convincing, but was not the shiny bronze colour which I wanted.
Maybe a problem with the LG2 bronze ingot? Something else?
So, I telephoned the supplier, Clingcast Metals, Sydney. Paul answered the phone, and he knew exactly what I was describing. “copper oxidation. Did you add 15 phosphor-copper shot to the charge? (The “charge”, I gather, is the crucible loaded with bronze pieces, for melting?)
“No. What is 15 phosphor-copper shot?”
“Small metal balls. If you add a tiny amount to the charge you will avoid the surface oxidation which you are experiencing, AND it will make the bronze melt thinner and run more freely.”
“Great. Where do I get it.”
“Oh. We have heaps here.”
“Would you post some to me?”
“Sure. How much do you want?”
Quickly thinking….”Maybe a kilogram”.
“That will last you a lifetime. $15, plus postage”.
“Fantastic.” – but I can’t find my credit card. Darn.
Paul. “I will post it. Just pay us when you get around to it.”
So, no more casting sessions until the 15 phosphor-copper shot arrives.
And big kudos to Clingcast Metals.
Meanwhile, I am making up trees and moulds.
Also meanwhile, I noted that Banggood have a special on melting furnaces which seem identical to the one for which I paid $425, and I have seen advertised at up to over $500. Banggood are asking $AUD290 + $20 p&p. So I have ordered one, as a spare. If you are interested I suggest that you check out this special, ASAP.
p.s. 2 Sept 2020. The 15% phosphor copper arrived today. I found this info on how much to add. I think that my 1kg will last a lifetime.
15% Phosphor Copper Shot is use as a deoxidizing agent for copper, brass & bronze alloys. It will also increase fluidity and inhibit gas porosity. The shot we carry is approximately 1/8″ x 1/32″ in size.
Melting of brass, bronze and copper should be done quickly in a slightly oxidizing atmosphere. The crucible is removed and surface is skimmed, then phosphor copper shot is added by simply dropping it onto the surface. The reaction is very visible. The metal will become fluid and bright. Allow a minute or two for the reaction to complete, and pour.
The phosphorus is a reducing agent (deoxidizer). This product must be carefully measured so that enough oxygen is removed, yet a small amount remains to improve fluidity. Too much phosphor can cause the melt to be so fluid that it leaks from the mold and penetrates the sand. A little goes a very long way!
The primary reason to use the shot is because the molds aren’t filling well or have gas porosity problems. The literature suggests that 1 ounce of 15% copper phosphor shot be added to 100 lbs of metal. A teaspoon of shot weighs about 1 ounce.
Start with these amounts: 100 Lbs add 1 Tsp. 50 Lbs add 1/2 Tsp. 25 Lbs add 1/4 Tsp.
For lesser amounts start with about 1 to 4 granules per pound of metal
One to 4 granules per pound of melt! It really will last a lifetime. My crucible will melt a maximum of 3kg/6.6lb.