Formulating An Exceptionally Refractory Clay Body.

[Tyler Miller]

I have an upcoming project that requires a high temperature vessel for melting and slow cooling.  Normally I would use something like a silicon carbide crucible for this, but the nature of this particular application requires me to break the crucible after every use.  With this in mind, I've been toying with the idea of putting together a clay body to make single use crucibles.  I'm bad at formulating clay bodies, so I thought I'd seek some advice here.

 

I have a variety of materials available to me already, a few different kinds of kaolin, fireclay, as well as mullite, kyanite, and the usual feldspars etc.  My initial thought was to do something like a 50/50 mix of kaolin and kyanite, with perhaps an addition of 2-5% feldspar to soak up a bit of the extra silica.  Is there any reason this wouldn't work as a body.  Is the feldspar a good idea? Is it something that will cause problems rather than solve them?  I don't care about cracking on the cooling cycle as long as it holds together on the way up and for the two-hour soak at temperature.

 

I know this isn't a usual clay body formulation and it's not quite relevant to what most do, but I thought I'd ask and share my results here as a (hopefully) interesting and instructive exercise in engineering clay.

[PeterH]

Tyler,

 

I believe that Hessian crucibles were traditionally used for this sort of thing, although I cannot supply a recipe.

 

IIRC 'Pioneer Pottery' by Michael Cardew has a section on DIY kiln furniture, which might have something helpful

(including advice on multiple firings to develop mullite structures?).

 

How high a temperature do you want to use. What controlled facilities do you have for crucible preparation.

 

Regards, Peter

 

http://www.sciencedaily.com/releases/2006/11/061123120134.htm

http://www.rsc.org/chemistryworld/News/2006/November/23110602.asp

http://www.academia.edu/820547/Mass-Produced_Mullite_Crucibles_in_Medieval_Europe_Manufacture_and_Material_Properties

 

 

PS Silly me, it's wootz steel isn't it.

 

http://www.bladesmithsforum.com/index.php?showtopic=4541

... contains the recipe

EPK 40
Calcined Kaolin 20
Tennesee Ball Clay 20
Grog 10
Flint 10

Mix with 500 g water per 1000 g mixture.

 

For a flavour of authenticity

http://www.swordforum.com/forums/showthread.php?75630-Clay-Crucibles-and-design-for-wootz-poulad
 

[Tyler Miller]

 

PS Silly me, it's wootz steel isn't it.

 

Yes, yes it is.  Thanks for the articles, the hessian crucible compositional analysis in the third link you post has helped me greatly.  I've used my ordinary gas-fired kiln for producing crucibles before, I was going to do the same again this time.

 

I had seen that particular recipe on The Bladesmith's forum--I post there regularly--but I wanted some more input from ceramicists about what makes a good high temp crucible--from a more theoretical perspective.  You've given me a lot of good information, now to use it.

[PeterH]

Probably following the same path that you have already trod.

 

Another recipe

http://www.artandknife.com/crucible.html

 

Packing your crucible (I hadn't realised about the layer of glass)

http://ceroart.revues.org/2557

 

Simple crucible forming

http://www.youtube.com/watch?v=E3my6-nxFjM

 

Making a graphite crucible (well graphite-clay)

 

Do you have access to a kiln to fire these? I'm certain it will greatly improve their quality.

 

Regards, Peter

[Biglou13]

Funny I read your post early today.

I just watched +ulfbehr+t documentary and thought ... Ingot......

 

My " making clay" brain went to work immediately.

I thought epk, and something to hold it together.... My guess wasn't too far off seeing recipes. Epk being cone 35.... (And I consider it a local clay)

Then I thought forced air/ power burner. (Which I have most of parts for, short gas plumbing parts) ( sure beats pumping the bellows)

Simple "kiln". 2 chamber "beehive-like" hard brick kiln. ( the bricks are in my yard), refractory mud. small hardbrick chamber for crucible, followed by space then beehive....

Pyrometer

Did I tell you I like sharp objects (knives). (Attended FMA seminar today, by renowned authority)

I then thought ....too bad your not closer 'cause I'd help you and trade you use of all my parts for something sharp.

The formula for the crucible refractory mortar....... Has Left me wondering???? Guessing......

What did they use for refractory material in 300 b.c. In Damascus, or 1000 AD for ulfberht. Since Edgar wasn't around yet.???

 

Ramble off.....

 

Let's know how this metallurgic and ceramic marriage works out.

[Tyler Miller]

Peter, thanks for posting those links.  I've come across them before, but it's good for easy reference to have them in one place.  Clay graphite crucibles aren't ideal because of the carbon migration that can take place from the crucible wall to the steel.  1 or 2 grams of carbon can throw off the entire thing.  I know a few smiths who do use them, but I'm a big fan of minimizing variables.  I'm too poor for slightly random results.

 

 

 

Biglou, kaolin was definitely known and used for the +ulfberht+ and wootz.  Theophilus refers to it in his chapter on Glassblowing by "use white pottery clay" for the glass batch crucibles. I think the development of hessian crucibles is more in the unique firing conditions under which mullite was formed.  But that's just a supposition on my part.  I'm pretty sure wootz crucibles resembled Hessians in composition, though.

 

I could put you in touch with a number of smith's a lot closer to you that would likely take you up on that offer.  I know of at least four in Virginia, the Carolinas, and Tennessee.  Drop me a PM and I'll see about hooking you up.

 

In the meantime, if you wanna have a little fun with something clay based and capable of creating something pointy (as well as something WAY more easy to handle than moody, cracky wootz), check this stuff out:

 

http://www.bladesmithsforum.com/index.php?showtopic=11327

 

http://www.leesauder.com/pdfs/furnace%20construction.pdf

 

http://www.leesauder.com/smelting_research.php

 

Knowing what you do about making clay, I would say you could dial in a furnace composition to get ideal slag production for some really slick bloomery steel--aka tamahagane.