Before the time I realised that working on gun stocks was more a thing I was suited to I did try colour case hardening. After a number of tries things went well, then a cracked receiver made me rethink things along the lines of this is not for you. Though I am one of those people who will research a subject as far as I can before I start and a long time after the initial idea has physically been superseded buy something else more interesting. also I am a compulsive note maker not to mention a working diary.
Now if there are secrets to colour hardening I dont know of any but there are a few not so talked about things that go to making a repeatable signature look to the finished work, with her emphasis on repeatable. But the not so good part is that you have to experiment to get your signature finished look.
I used 6" X 4" X 1/4" mild steel plate for experimentation. Now the carbon packing we all know is charcoal and bone carbon but that is just a common mix you can purchase. But like an artist there are other things you can add to the basic mix that does alter the final look of the case hardened metal. Here are some that can be added, carbonised Leather, powdered anthracite, In times past carbonised Ivory turnings, Carbonised hoof and horn. and one I found added some lighter colours, carbonised blood fish and bone garden fertiliser, though it does give off some not so nice smells when heating to carbonise it.
The quenching water, Rainwater is best because it has no minerals to complicate things or change the finished colours. At the point before you drop in the hot metal stir the water violently or better still have air blowing through the water to churn it. And finally it was said that adding Salt Peter (Potassium Nitrate) to the water would brighten the finished colours.
I hope this may be of interest though it is all experimentation for quantities, one thing I omitted was making your own charcoal from the same wood every time removes another variation that could make things non repeatable.
