Exploring local materials in the development of translucent bodies: John Shirley discusses the results of his translucent body research in South Africa.
AS A CERAMIST WORKING 1N TRANSLUCENT BODIES in South Africa I was interested in developing these bodies using local materials. What I have found with our kaolins is that they tend to be contaminated with trace elements of iron. This does not bode well for the ceramist wanting to make porcelain bodies but is almost an advantage if you are investigating the use of bone china. The bone ash in the body (almost 50 percent) bleaches the iron present and seems to render whiter bodies with slightly impure kaolins than the very pure ones. These are, if not whiter than the purer kaolins, then at least of equal whiteness. This is the material that my recent research has been around.
For the purposes of this article I will concentrate on the casting slip variant. Due to the bleaching action of the bone ash one could probably get away with higher percentages of plastic ball clays than one could in a porcelain body, though one would need to keep the bone ash content at a minimum of 40 percent. Instead of bone ash which, as the name implies, is made from calcined bones, I use tricalcium phosphate which is a naturally occurring material, both whiter and purer than the traditional bone ash. As a starting point in the making of a bone china body I would use a traditional recipe which (in percent) would be:
Bone Ash 50 Kaolin 25 Felspar 25
The first tests involve about 50 grams of these mixtures using whatever kaolins are available locally.
* Simply dry mix them, add sufficient water to make a workable mix and fashion small flat discs from the mixes.
* Once dry fire the pieces to about 1250[degrees]C.
* After firing, see which test gave the whitest and most translucent mixes.
* The next step is to use the chosen kaolin and repeat the process above varying the felspars that are available, for example: potash, soda and China stone and so forth. Fire as above and again select your best result.
* At this point you should have a translucent, white test piece.
* The next stage is the conversion of this body to a casting slip.
* You will now need to weigh out a mix of 1000 grams. Start your tests with 40 percent water, (400 ml) to which 3 grams of sodium dispex has been added.
* Mix well and pass through a medium sieve at least three times. After this, check the specific gravity (SG) of the mix. The SG could be slightly heavier than usual and I suggest be tween 1.8-1.9.
To measure the specific gravity measure 100 ml of the mixture and weigh it. It should weigh between 180 and 190 grams. If it weighs more, then more water can be added. Conversely if it weighs less, then more of the dry mixture must be added. Once the specific gravity has been adjusted, the fluidity can be increased, if needed, by adding dispex to the mix drop by drop.
To start with your casting I suggest that you use simple, one-piece drop out moulds. The casting time will vary but it surprisingly shorter than with conventional porcelain casting, often as little as 30 seconds is enough time to make a cast.
This article set out to introduce you to the development of a bone china body with materials available to you and there will probably be further experimentation needed before you create a body that suits all of your needs. You will find that this body will have minimal green strength and it is necessary to bisque fire at about 1000[degrees]C before fettling by sanding.
You could try replacing some of the kaolin with bentonite, up to 6 percent, to improve the green strength but I still prefer to do all fettling on my work after bisque using Dremel tools and sandpaper, wet and dry. It is important to wear a mask during any sanding processes to prevent the inhalation of any fine dust.
Sometimes things that first seem to be catastrophic are just the opposite. I had been using a body for a number of years and was informed that the kaolin I was using in it was no longer available. This forced me to look for alternative sources. A new kaolin that I came across, which I would not have done had I not been forced into the situation, yielded results far superior to the original one I was using.
Traditionally Bone China has been used as a casting slip, but I have also looked at the possibility of making a bone china throwing body. Although this would set many challenges in itself it would be a worthy area for further investigation.
John Shirley is currently Programme coordinator and lecturer in Ceramic Design at the University of Johannesburg. He has been a practicing ceramist since 1970 and began working with bone china in 2002 while completing his B Tech in Ceramic Design. In 2008 he won a merit award in the Ceramics South Africa National exhibition for his work in bone china.
All photos by John Shirley.
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|Date:||Nov 1, 2009|
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