Kurouchi Clay

Johspr1

Johspr1

Joined
Feb 9, 2014
Messages
17
Hi All,
A quick question regarding Kurouchi clay mixture: I have heard a mixture of Satanite (some sort of refractory coating) and clay can be used as a pre-HT dip to get the black Kurouchi scale. Has anyone had experience with this finish? I know people have their own blends, but what might be a good place to start? A certain type of clay? Anything would be helpful as I have no idea where I might even get my hands on something useable.

Thanks,
John
 
Kuro uchi* ( literally "black inside", but meaning "black forged") is a "brute de forged" finish left on Japanese blades as a stain resist area on the upper bevel. It is just the "as forged" surface that has the black decarbed and darkened metal left rough. The lower bevel is ground to a smooth and sharp edge. This finish forms as the steel is repeatedly heated in forging. There is no simple way to add it without forging or repeated heat and cool cycles. The clay does not cause this finish.

The clay mixture is used in HT to get a hamon, or just to get differential hardening of the edge and spine is often a blend of refractory clay ( like satanite), and fine field or potters clay. Other things like ash are sometimes added. The clay wash coat also protects the blade from excessive decarb.

Most smiths use just plain satanite or similar things like APT-36 refractory cement. Mix it thin as pancake batter and don't put it on any thicker than a wash coat. the spine can get a slightly thicker second coat, but you still want it no more than 1/16" thick at the spine.

*Uchi is "inside" ( as in inside the house) or "home/house", but can mean all sorts of things when hooked to another word. With the word kuro (Black) it refers to the surface left after it was inside the forge ( home of the blade in forging).
 
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Stacy,
I have seen some japanese knife makers dip their blades in a thin, watery clay that they let dry right before heat treat. If this does not have any effect on the amount of blade "blackening," what is the purpose then of this process? I am not referring to differential heat treating in this instance as the whole blade is quenched.

Thanks,
John
 
As Stacy mentioned, the thin coating of clay helps prevent decarb by shielding the steel from oxygen. I believe it also helps even out an otherwise rough quench. The clay slip can either be applied with thicker clay for a hamon, or by itself on the whole blade.
 
Thank you gentlemen, that is helpful information. Better switch over to forging if I want to get the finish I want. I'll try not to set myself on fire...again.

Thanks,
John
 
This might not be what you're looking for, but I used a cold blue/bleach etch on my last knife before grinding and it left a pseudo forge finish look on it. Different etch times can yield different textures (this was etched for about 2 hours)
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As James was showing, there are several ways to get a dark rough upper bevel for a pseudo kuro uchi look. Etching in FC, Bleach, using Parkerizing, etc. will all darken the area.

Also, a partly ground blade can be cycled from 2000F down to black heat about five or six times to get the forge finish. After that, soak the blade in dilute Sodium Bisulfite or vinegar overnight to remove the hard scale. This should leave the rough decarb "skin". Etch as desired to blacken more. The lower bevel is then ground to about 90% of the desired thickness and HT is done.

As pointed out, the slip/wash coat of clay is to protect the blade surface from oxygen and also to help even out the quench. Blades with a clay slip crack and warp much less than ones left bare.

Many Japanese blades have the black surface enhanced with an application of beeswax and turpentine ( often called a "blacksmiths finish" in the west). This is applied to the hot blade when the blade has cooled down to about 700-800F. It will smoke, and possibly burn, like crazy, but when the blade has finished cooling, there will be a black "varnish" baked into the rough upper area. Any of the black varnish on the other areas of the blade will be removed in grinding and sharpening. Rubbing in the black with steel wool makes for a nice sheen.

In other cases, a black paint/varnish is applied and rubbed into the rough area. The excess is wiped off, leaving the recesses blacker. This is usually done after etching deeply with FC to get as dark a basic surface as possible. Some of these are epoxy/resin based and are baked at 200-300F to make them really tough.

Additionally, a faux- forge finish is applied to a profiled blade by heating the blank to 1600-1800F range and hammering or pressing a rough or textured pattern into the surface. The lower bevel is ground away in finishing the knife, leaving the textured surface on the upper area. This is how the popular "cross hatch" or "squiggled" upper bevel is attained on Japanese sporting knives, as seen here:
http://www.bing.com/images/search?q...4D91F61ED80F3E6955A087C865E582294&FORM=IQFRBA

There are old traditional ways to get those squiggles and hashes by etching, but modern production procedures have pretty much replaced that. The steel sheets are hot rolled through rollers with the pattern on them, and the blanks stamped out of the sheet. The rest is just stock removal. While sold as a " forged finish", it is no more forged than a "drop forged" shovel is.
 
Thanks guys, I just put a small blade through multiple heating/cooling cycles and it is currently in a vinegar bath. Thanks for all the techniques, I'm going to experiment with them all.

Thanks,
John
 
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