Dave –
Thankyou. I’d never thought that small amount of CO2 could case-harden anything, but I suppose the volume of oxygen present is small anyway, certainly far less than would be attacking the work if heated in the open.
I’m glad you suggested it Nigel, because I’ve learned something. As Jack says “Oxygen is usually a much worse problem than CO2. ”
My reading on Muffle Furnaces is they were developed specifically to keep combustion products away from the item being heated. Plenty of circumstances where clean heat is essential, including warming my breakfast croissants!
But having said that, case hardening is a common workshop requirement. As Carbon is already part of the process there’s no harm in having CO₂ in the furnace and advantage in reducing Oxygen because it attacks steel. Therefore burning Charcoal as you suggested is a good idea.
CO₂ not good in other examples, especially if made by burning Carbon in the oven. To easy for a jeweller or potter to ruin an enamelling job by getting Carbon smudges or ash in what has to be a perfect surface.
Borax or other flux may also be good or bad depending on the circumstances. Whilst flux keeps air away, it’s not good at stopping solids reaching the job.
Muffle heat has to be clean in the context of the job, which means CO₂ and Flux might be either great or terrible! I now think burning charcoal in the oven will help when Carbon is less damaging than Oxygen, but otherwise setting a fire inside what’s meant to be a clean environment is risky! When cleanliness really matters, an industrial Muffle Furnace operator is better placed than us because he can get rid of Oxygen, water vapour and dust simply by flushing out air. Argon, Nitrogen, CO₂ and other clean gases are cheap provided they’re bought in bulk; no problem to industry. Sadly, most hobbyists have to buy gas in small quantities, and that’s very expensive.
Sonic has since confirmed he’s not planning anything demanding more than ordinary cleanliness. Case hardening rather than Jewellery. I don’t think he can go far wrong whatever he does!
Most educational this forum, thanks again,
Dave