What a difference the choice of clay makes.

[George Lewter]

When you evaluate a glaze you really need to test it on different clays. The following two pots were glazed the exact same way (spraying Jen's Juicy Fruit as a base glaze and then lightly spraying Orange Red as an accent color on top of the JJF.

This is the combo on Laguna B-mix 5

This is the same glaze combination on Laguna WC608

Both were sprayed from the same guns and fired together in the same kiln load.

As a general principle white clays will allow you purer, more saturated colors than you can get with darker and speckled stoneware bodies.

[Tom Humphries]

Wow big difference, they both look nice though!

[Rodney Allen Roe]

Good demo.  I'll bet if you took a poll for which is liked best it would come out even.  Different but both nice!

[Tom Anderson]

Hi George:

I have been working for several years on clay formulation. The oddity being there are no clear set of formula limits, like what you find with glaze. In the last four months I have been testing iron, magnesium, and titanium limits in clay bodies. I would like to share those results for members to use as a comparison chart. From your photos above: I would estimate that WC608 has much more magnesium than the B-mix.

Bar 1  MGO 0.33   FE 0.32            Bar 2 MGO 0.89   FE 0.36           Bar 3  MGO  0.78    FE  0.42              Bar 4  MGO  0.42  FE  0.36

Bar 5 MGO 0.63   FE 0.32             Bar 6  MGO 0.49  FE 0.40             Bar 7 MGO 0.36       FE 0.86           Bar 8 MGO 0.42  FE 0.74

Bar 9 MGO 0.97   FE 0.91   TiO2  3.77        Control Tile- top  MGO  0.18   FE  0.32

All test bars fired to cone 6 in oxidation without glaze.

Simply fire a test bar of your clay to maturity without glaze and compare to the chart above for results. It will give you a general sense of your clay body impurities. Typically, titanium is an almond range, magnesium is a grey range, and iron darker almond to light tan.

Remember: your glaze can leach these elements, just like it does alumina.

Tom Anderson

[Joseph Fireborn]

Tom,

Bar 9 explains why I get a lot of variation out of my stoneware compared to my porcelain.  I thought it was the iron that was giving me the results, but it looks like it might be the iron and titanium. Would make sense now why I get a lot of really interesting results on stoneware and a much more controlled result on porcelain. Interesting stuff. 

[Tom Anderson]

Hi Joseph:

It is almost an automatic response among potters to think they did something wrong in mixing their glaze when results vary as George shows above. If you have differing results on the same clay: then that holds true. However, if you put the same glaze on two different clay bodies: then you should expect differences.

The other issue is potters assuming because a clay is white: it is cleaner- a bit of a myth. C&c ball clay is considered a white clay, but it has 0.93% iron (FE). OM4 ball clay is used often in glaze recipes; and yet it has 1.10% FE. McNamee kaolin is darker than C&C, but only has 0.33% FE. As you have heard me long preach the benefits of NZ kaolin because it is the purest of all clays. NZ has 0.25 iron, 0.05% titanium, and 0.00& magnesium (MGO). I use it in custom clay bodies, but I also use it extensively in glaze recipes. It has excellent alumina and silica ratios as well: a perfect solution for COE glaze issues. Yes it is more expensive, but cheaper than dismal results.

Tom

[Marina Reijsmeijer (Kleierij)]

Interesting! I am a coward. I always use the same clay. This makes clay recycling much easier too. Grès St Amand, fires light pink in oxidation.

[Tom Anderson]

Marina:

All the samples above are porcelain. Few years back I fired 50 lbs of earthenware: it has a very pastel rose/pink color to it. I suspect that was iron as well. Pleasure to meet you as well.

Tom

[Tom Anderson]

Plasticizer question? Do you prefer macaloid or Vgum? I have my own preference, wondering what experiences members have had?

Tom

[Norm Stuart]

There's a range of Veegum and Bentone products, and each product varies naturally depending on the ore they're mining.

While we typically use Veegum-T, but even a small amount of Veegum-T denataures a glaze we have - a 50 pound bag of CM-941 Frit which is a complete Cone 06 Red Glaze colored with cadmium sulfide and some lead.  For some reason 0.5% of Veegum-T turns this black.

Bentone-EW in a quantity of 0.5% suspends this frit while darkening it only slightly, so it has that use where Veegum fails us. I bought the Bentone-EW from Trinity in Texas. At the time the product information indicated it came from China.

I suppose the real question is how is a complete glaze frit intended to be used?

Tony Hansen at Digitalfire has these photos:

[Tom Anderson]

Norm:

TY much for the quick response, nothing like tests to confirm. I buy very little raw clay materials from typical clay arts dealers. I buy technical grade kaolin, silica, Nep SY, whiting, and a host of other products from sources outside our trade. I have formulated a clay  specifically for tile work: and tailored for crystalline glaze. Sodium and magnesium are under 0.20%, and iron under 0.35%. With technical grades, I can order a range of particle sizes in the same body, with differing analysis. I have used the T grade Vgum, but was considering the Pro grade which is two grades  above T. Was thinking of adding some plasticity for throwing down the road.

Tom

[Marina Reijsmeijer (Kleierij)]

Hi Tom

Nice to meet you. My clay is a French stoneware clay and with enough oxygen it fires pink, on the inside of lidded jars it fires whitish. I am sure there is iron in it, it shows as a ring sometimes above a glaze.

Marina

[George Lewter]

Tom, gotta say I admire your dedication in mixing both your own clay and glazes. I suppose working with crystalline glazes is an incentive to know the exact formulation of the underlying body. That is something that is never revealed about any commercial clay body that I know of. I am more concerned about the degeneration of my shoulders than the composition of the clay body, so that is a whole area that I am happy to avoid with its added physical work.