Archives for the month of: April, 2016

I recently spent some time flipping through the Laguna Clay catalog and found a few kaolin’s I had not worked with before. Me being who I am I wanted to see how they acted and looked in a clay body. The clay body that I use is a variation of Victor Babu’s porcelain recipe. I currently do not have a clay mixer accessable to me and have been making all of my clay as a slip and then allowing it to dry out. The reason that all of this is important to me is that I have found most commercial clay-bodies inadequate for what I want my clay to do. I am not looking for translucency in my clay. I simply want a clean white durable clay-body that looks good in my soda firings and my electric firings. I have found that most commercial porcelains that I have tried tend to warp and deform under the tensions I place on them with such a wide range of glazes and unglazed surfaces. For example I recently had a large 18″ jar that was thrown with Ardvark’s Coleman Porcelain. The patterning on the surface included several areas of unglazed decorative elements. The tension of the satin glaze I used caused the jar to warp into lobed sections. This type of thing drives me bananas! So my clay is generally less glassy, more opaque, and slightly courser to help prevent this.

Like I said previously the recipe I have been using for the last 7 years is based off of a Victor Babu recipe that I think I orignally picked up from a Matt Long hand out. I have tinkered with it over the years but it revolves around this base formula.

Bloomer /8-10 Porcelain for Oxidation & Soda Neutral

Grolleg Kaolin 28 lb

Tile 6 Kaolin 27 lb

Custer Feldspar 20 lb

Silica (325mesh) 22 lb

XX Sager Ball Clay 2-3 lb

Alumina Hydrate 1-3lb

Mullochite (200 mesh) 0.5lb

Mullochite (120 mesh) 0.5 lb

I generally make this clay a little on the stiff side so that it creates coarse texture when I facet it.

For these tests and to get a better idea of the properties of each of these kaolin’s I used it for 100% of the kaolin in each test. I was able to get test into a studio cone 10 reduction firing and a studio cone 10 reduction soda firing. I have also tested each clay for throw-ability, and workability, as well as shrinkage.The kaolins tested are MCNAMEE, KINGSLEY, IONE, NEW ZEALAND, & ENGLISH STANDARD

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The last tile here is a blend of 50% McNamee and 50% Grolleg. This photo is from /10 Reduction firing.

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These tiles were fired in cone 9 electic on the left. The tiles on the right were all in the same spot in a cone 10 reduction soda firing. Both columns are in the same order as the previous photo.

Notes on each in my clay body:

 

McNamee – Slightly short and has low plasticity. Was acceptable for pulling handles and faceting but not great. Very creamy and slightly yellow in color in reduction. 11% shrinkage. In cone 10 reduction soda firing the tiles were slighly mottled and gray.  In cone 9 electric the clay is slightly lighter and less yellow.

Kingsley – This test was very pleasant to throw with. It was very smooth and plastic.Pulling handles with this clay was great. Although it threw pretty well it didnt behave very well when thin. 12.5% shrinkage and a slightly creamier white than the previous tile in both reduction and in oxidation. In soda firing the tile did not take a lot of carbon trapping and remained closer to its actual color.

Ione – I think that this kaolin is more frequently used for slip casting but I thought I would try it anyway. In this recipe the ione kaolin is very soft and has a low tensile strength. The plasticity of this clay is lower than anything I’ve ever experienced before. It did not wedge well, pull or coil well and did not like to stand up. The shrinkage rate on this clay is 14.5%. It has a very low density to the feel of the clay and is generally unpleasant as a throwing body. In terms of color it is very similar to the kingsley kaolin and is a very clean creamy off white in both reduction and oxidation but is slightly warmer and toastier in reduction. In the soda firing it seemed more susceptible to carbon trapping and had a slightly mottled and speckled gray quality.

New Zealand – This stuff is pricey and is actually a halloycite. As best I can tell right now this is the whitest primary clay out there and it tested very nicely for color in my tests. Of all of the clays I tested this was the most thixotropic, so much so that I could barely use it. the cylinders and bottles I threw with it didn’t want to stand up, the handles I pulled flopped over, and the moment I thought about making it thinner it succumb to gravity. All of the tiles were a bright white. The soda fired tile was bright white with no carbon trapping at all. The shrinkage with this kaolin in my clay body was 13.5%.

English Standard – This was a very nice kaolin to work with and had good plasticity. It threw well, pulled well, and felt nice. The cylinders and bottles I threw were all very easy to make and held together when thrown thin. This kaolin in my clay body in reduction was a blue gray, not my favorite color but acceptable. In oxidation it is a slightly whiter color with some blue gray tones. In soda it grabbed a ton of carbon and was gray all over. This clay has a shrinkage rate of 13%.

The last tile pictured blended two kaolins to see what the result was. For this tile I used the following recipe:

Grolleg Kaolin: 27.5

McNamee Kaolin: 27.5

Silica 22

Custer Feldspar 20

XX Sagar 2

Alumina Hydrate 1

Molochite 1

This clay was surprisingly gray in reduction. I thought that given the whiteness of grolleg and the yellowness of the McNamee that it would be completely different than this. It was very plastic and pleasant to use. It pulled pretty well and has a nice workable strength. The shrinkage is 13% and I unfortunately did not get to test it in soda yet.

 

What am I taking away from this? There is a reason we use the primary kaolins that we do. Many of these kaolins might be good in specific recipes designed for specific purposes. here is how I would rate these tests. My first choice for the most pleasant to use would be the Kingsley, followed by the English and McNamee. The New Zealand kaolin is so floppy and thixsotropic that I have no interest in trying to make it work despite its beautiful white color. The Ione kaolin is unsuable in my throwing body. I found myself wishing the for the Kingsley and English to have the whiteness of the New Zealand. I think in the future I will stick with grolleg.

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This series of tests was inspired by Tom Jaszczaks workshop at Santa Fe Clay last summer. I had been firing a few select items for the last 3 years in electric kilns to cone 1 or 2 and was curious about how soda firing at this temperature would work out. I already had a clay body that I had tested and used at that temperature and knew it wouldn’t bloat or deform (Laguna R2 or R2 with grog). I had no idea how any of the previous cone 1 tests I developed back in 2012 & 2013 would fare in a reduction soda firing. During the workshop I was able to get a few of the glazes I had previously made tests for into the kiln. Unfortunately my previous glazes either blistered or melon skinned and were not generally happy with the atmosphere. These glazes were predominantly nepheline syenite, generally around 60-70%, and frit. So last fall I sat down and picked a few lowfire glazes out of my recipe lists and began making small test batches. I selected glazes to contrast each other. I selected glazes that were opaque, clear, satin, or gloss, but of more interest to me was the predominant flux of each base. I tried to choose a diverse selection of fluxers. The main categories were: Frit (3110, 3124, 3134, 3195, and combinations of the previous), Lithium, and Gerstley Borate. There were also a few recipes that paired Soda Ash or Borax with other melters but they were never a primary component of the glaze.

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I was expecting to see more results that were either melon skinned or blistered. There were really only a few tiles that didn’t look like they should be used in this type of firing. The most notable of these was the Moonalis Chartreuse (which is a nepheline syenite heavy recipe and bubbled and blistered quite baddly) and Sar Guntel Young II (which is 70% Lithium carb and is beautiful but shivered off of the R2 terracotta). There were a few others that blistered and you will see them in the photos I am sharing in this post.

The most surprising thing for me was the number of clear bases which turned red. I am not sure whether they were picking up copper from the atmosphere, iron from the claybody, or a combination of both. When looking at the photos of the tiles you will see a number of base glazes that have a mauve-iron red cloudiness to them. They do not have any colorants in them but in my next cone 3 soda firing, I will test a few of these with colorants to see how much the red coloration changes.

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I also selected a variety of slips for this firing and even a few ball clays used on their own. I was curious about a few standard midrange and high fire slip recipes as well. I was pleasantly surprised by a majority of these slips especially the ball clays and high fire slips. All of these slips were applied to bisqued tiles and could stand to be thicker. That is something I will test in future firings.

Overall I am very pleased with the outcome of this firing and with the majority of my tests. Enjoy and feel free to use any of these recipes.

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