Recently New Mexico Clay Customers had a problem with a clay body (High Hills) we have been making for 14 years.

HIGH HILLS is our version of the popular Missouri fireclay goldart ball clay and grog type stoneware. It is a very forgiving and throwable clay for production potteries. Tan to gray-brown in reduction, medium texture.

Cone 10 Shrinkage: 14% Absorption: 0.1%   Penetrometers: 7

We previously had to fix a problem with iron or manganese spots caused by the fire clay Hawthorn 35. This clay is used by various companies to make refractories, cement kilns and other industrial processes. Only 1% is used to make clay for potters. The spots were caused by the fact that the fire clay was unscreened. The purpose of a fire clay in a clay body is to provide tooth or texture.

We are now using a screened (40 mesh) Hawthorn.

The new problem mainly effected people who fire in gas kilns and do a heavy body reduction. These Potters experienced shivering and dunting; probably caused by a change in the chemistry of the fireclay.

The samples the potters brought in showed signs of Black Coring caused by over reducing iron; changing red Iron oxide to black iron oxide or worse-iron silicate. Large amounts of coal and other organics in the clays can exacerbate black coring, so I checked the raw materials for excessive Loss on Ignition.

Everything seemed perfect no problems, L.O.I. was lower than expected for all the clays.

Test of raw clays in High Hills
	Initial weight	Dry*	loss	Fired#	LOI	color
OH #5	294	293.1	0.31%	265.5	9.4%	creamy white
Goldart	294	290	1.36%	263	9.3%	creamy white
OM4	294	291	1.02%	263.4	9.5%	creamy white
Hawthorn 40	294	288.5	1.87%	257.3	10.8%	peach
	grams
	*Bake at 350F for 4 hours
	#Bake at 1000C hold 1 hour 1832F

Large amounts of iron also cause black coring but as you can tell from the clay descriptions this is a light colored clay body.

James Jarrett of Imerys (A Giant Clay Company) took pity on our plight and ran some tests on all the ingredients and the finished clay body.

						New Mexico Clay
						2011-19
Sample id					OM #4	Goldart	# 5	Hawthorn	High Hills
					K-T Clay		OHCC	Fireclay	C 10
								40 mesh	12/29/10
	Received				05/31/11	05/31/11	05/31/11	05/31/11	05/31/11
Chemical
	Si2O3			%	59.3	56.0	59.1	54.1	57.7
	Al2O3				27.8	29.8	27.7	31.2	29.3
	Fe2O3				1.19	1.26	0.83	1.93	1.64
	TiO2				1.81	1.46	2.25	1.37	1.34
	CaO				0.14	0.37	0.12	0.32	0.33
	MgO				0.19	0.49	0.13	0.30	0.33
	K2O				0.45	1.44	0.52	0.96	1.15
	Na2O				0.11	0.11	0.11	0.11	0.11
	L.O.I.				9.2	9.9	9.2	10.5	8.2
	Carbon				0.186	0.102	0.057	0.048	0.085

 This showed nothing wrong.

High Hills clay Reduced
 in Miya Pottery’s Giel kiln at 7000’

Same piece on edge Note the Obvious black core.


EDGE        Glaze      Air


Nothing wrong… So I called a ceramic engineer, Jeff Zamek.

Jeff agreed to try to figure out the problem, he studied the formula and interviewed the people who had problems.

He suggested a few changes in the clay body; we have made these changes and now call the clay High Hills II

My conclusion is that having too much fireclay in the formula coupled with long reducing firing allowed for excessive Cristobalite production. Cristobalite changes its shape around 200°C (392°F). This can cause dunting and shivering.

Some of these steps can help. (this from Laguna Clay)

1. Fire slowly enough 1290° to 1650° F to give the carbon and sulfur in the body time to combine with oxygen and work its way out of the clay body. The burnout process must be done slowly and completely before the glaze begins to melt. In oxidizing atmosphere!2. Provide an oxygen rich atmosphere. In electric kilns, use plenty of ventilation in the bisque fire. If your kilns are densely loaded, use a downdraft kiln vent. In gas kilns, keep the kiln in oxidation from 1290° to 1650° F.
3. When using iron-bearing clay bodies, fire up to 2100° F in complete oxidation to allow for proper and complete burn-out of the organics; then, if desired, begin the reduction process.

Further reading:

Putting the Fire in Clay by Jeff Zamek
http://www.ceramicindustry.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000000159771

How to Solve the Problems of Body Cracking and Glaze Popping in Stoneware Bodies  by Maynard P. Bauleke  see..
http://www.kgs.ku.edu/Publications/Bulletins/211_4/bauleke.html