Recommendation? Must I polish between hardening and tempering?

[Hugobenedicteddie]

Hi Lads,

I am aware this might be a stupid question, but I can't seem to find an answer anywhere, so I thought I'd better ask.

I'm making a few knives here in Ireland, and heat treating with a torch in a homemade kiln.
After the heating and quenching etc, the metal is obviously quite blackened, which is an appearance I quite like. So, my question is; do I have to polish the black off the surface before I temper it?
Ideally, I would like this blackened surface to be the final surface, but is it poisonous if I was to go using the knives for food etc then?

I realise this is probably entry level stuff, but I can't seem to find an answer anywhere!
Hope ye might have some light to shed on the situation.
Thanks a mill.

Hugo

 

[Stacy E. Apelt - Bladesmith]

NO, temper immediately after hardening. The sooner the better. Once tempered, the sand and finish the knife.

Any work done on a hardened but untempered blade could easily break or crack the blade.

 

[Hugobenedicteddie]

NO, temper immediately after hardening. The sooner the better. Once tempered, the sand and finish the knife.

Any work done on a hardened but untempered blade could easily break or crack the blade.

Thanks Stacy - my question then is, if I want to just leave all that blackness on the finished product, will it wear off over time? Also, if it does wear off, is it safe to use on food??

 

[Stacy E. Apelt - Bladesmith]

All the blackness - I would say NO.
Some of the blackness - YES.

The forge finish and black is often left off the upper bevel. It is called Brute-de-forge on western knives and Kuro-uchi on Japanese blades. The lower bevel to the edge is always sanded/stoned/ground to be smooth and bright. The rough finish isn't harmful as long as scale isn't flaking off, but it will trap bacteria more easily.

 

[weo]

Are you trying to temper with a torch? If so, you'll need to quickly sand off enough of the black to see the temper colors. However, this is not recommended as mentioned in other threads if you search for torch tempering.
If your kiln is able to hold a constant temp (+/-) 15-ish degF, then do as Stacy said. Not bragging, but my tempering oven that was salvaged from a lab only gets up to 450, but only fluctuates +/- 5 degF, so if you can find something like this, it's worth it because this is probably the most important part of the whole operation to make a functional tool.

 

[Hugobenedicteddie]

Are you trying to temper with a torch? If so, you'll need to quickly sand off enough of the black to see the temper colors. However, this is not recommended as mentioned in other threads if you search for torch tempering.
If your kiln is able to hold a constant temp (+/-) 15-ish degF, then do as Stacy said. Not bragging, but my tempering oven that was salvaged from a lab only gets up to 450, but only fluctuates +/- 5 degF, so if you can find something like this, it's worth it because this is probably the most important part of the whole operation to make a functional tool.

No, I am going to temper it in an electric ceramic kiln, which can hold its temperature far more accurately and for longer than my rinkydink torch one. Though the torch kiln works a treat for the hardening, it's the duration of the tempering process and need for a reasonably steady temperature that makes it unsuitable I think, you know? So the ceramic kiln should be perfecto

 

[weo]

Right on, have fun!