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Monthly Tech-Tip from Tony Hansen

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Glaze is bubbling with an encapsulated stain? It needs Zircon.

These two pieces are fired at cone 6. The base transparent glaze is the same (G2926B Plainsman transparent). The amount of encapsulated red stain is the same (11% Mason 6021 Dark Red). But two things are different. Number 1: 2% zircon has been added to the upper glaze. The stain manufacturers recommend this, saying that it makes for brighter color. However that is not what we see here. What we do see is the particles of unmelting zircon are acting as seed and collection points for the bubbles (the larger ones produced are escaping). Number 2: The firing schedule. The top one has been fired to approach cone 6 and 100F/hr, held for five minutes at 2200F (cone 6 as verified in our kiln by cones), dropped quickly to 2100F and held for 30 minutes.

Context: Mason 6021 Red Stain, G2926B, Firing Schedule, Encapsulated Stain, Cone 6 Drop-and-Soak Firing Schedule

Wednesday 24th November 2021

G2934 with pinholes on three cone 6 clay bodies

Three pottery mugs, two having glaze pinholes

This was a fast firing. The glaze is G2934, a silky matte. But that does not mean it is pinhole-prone, it has good melt mobility. The clay on the right is Plainsman Coffee Clay. It contains 10% raw umber, that generates plenty of gases during firing. The centre one is Plainsman M390, not normally difficult to fire defect-free. The left one, M332, should be the worst, but is the best! What is needed to fire these without pinholes? The drop-and-hold and slow-cool C6DHSC firing schedule. It is extra effort to program your kiln controller, but well worth it. If you don't have a kiln controller then by a little experimentation you can develop a switching pattern to produce the same effect.

Context: G2934, Plainsman Cone 6 Slow Cool (Reactive glazes), Pinholing

Tuesday 23rd November 2021

Why does Tony Hansen take months to unload his kilns?

I love making pottery, but I love the technical side more. I searched for all the test specimens in this load of cone 10 reduction ware first, then pushed it back in and forgot about it. For three months! I really anticipate the test results (I am developing and adjusting many of bodies and glazes at any given time). The data and pictures for them go into my account at, it enables me to compare the chemistry and physical properties of recipes and materials side-by-side. That teaches me which roads to abandon and which ones to pursue. My last kiln went back in for six weeks, so things are getting worse!

Context: The recipe mixing area of Tony Hansen, Physical Testing, Tony Hansen, Hansen

Monday 22nd November 2021

Was that batch of frit 3195 really bad?

After a customer experienced blistering with a transparent glaze on a terra cotta body, suspicion was raised that the batch of frit of bad. Ferro even admitted there was an issue. But we decided to test, sampling about 20 of the 50 lb bags and combining that to make these tests. Upper left is a GLFL test, if compares the melt flow of an old batch of frit 3195 with the questioned one, fired at 1750F. Although the questioned batch does run a little more, a more important difference is in the bubble development (look closer), perhaps there is some fluorine contamination. A GBMF test compares the two samples on the bottom left (10 gram balls melt downward onto a sample tile). Again, the questioned batch is melting and bubbling a little more. We made two glazes: Cone 6 GA6-B (top right) contains 20% of the frit (the left one uses an old frit batch). The cone 05 G1916QL1 glaze (bottom right, it contains 60% of the frit) uses the questioned frit and works well on both the white and red bodies. So, although it might not be working for some, we determined that the frit is OK! Although it is possible that only certain bags of the batch were bad, that seems unlikely given the way frits are made, all bags in a batch should be the same.

Context: Ferro Frit 3195

Thursday 18th November 2021

A cone 10R iron crystal glaze using only Ravenscrag Slip and Iron

Two mugs with iron crystal glazes

Ravenscrag Slip, by itself, produces a silky transparent glaze at cone 10R. It is an excellent base to which to add colorants and modifiers. This is a simple addition of 10% iron oxide (Ravenscrag Slip already contains 2% iron, making about 12% total Fe2O3). It produces a stunning crystalline fired surface on these two porcelains. We can call this a "Beyond-Tenmoku" (crystals happen because of more iron or a slow cooling rate than a Tenmoku). The 12% iron dissolves in the glaze melt during firing, but during cooling in the kiln, the extra 2% precipitates out to produce these surfaces. The iron also acts as a flux in reduction firing, greatly increasing melt fluidity. Take that last statement seriously: The iron is a flux, the glaze will melt much more. So just adding iron oxide to a glossy transparent will wreck your kiln shelves when it runs down off the ware. Ravenscrag Slip does not melt-to-glossy, it has just enough feldspar to fire to a durable surface, making it a more stable host for the iron addition.

Context: GR10-L, Iron-Red high temperature reduction fired glaze, FeO (iron oxide) is a very powerful flux, Same high-iron glaze. One crystallizes and the other does not. Why?, Ravenscrag web site

Thursday 18th November 2021

A step to prevent cracking clay mug handles

Drying cracks are opportunistic. They will initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure after trimming you will deny a crack a place to start. Of course even drying is still important, the water content of a handle should now be allowed to get ahead of that of the main body of the mug at any time. In the pictures on the right, two tools are being used to compress and round the angle at which the handle meets the wall of the mug.

Context: One way to avoid drying cracks on handle-joins of engobed mugs, Drying Crack

Monday 15th November 2021

Why do some clays split like this on throwing or forming?

A split on the lip of a bowl

An example of splitting, where a thrown clay develops a split minutes after throwing. This often happens at stretch-points with sandy or groggy clay bodies or those that have a wide range of particle sizes (e.g. native clays not ground to 200 mesh). The larger particles create networks into which water can penetrate and begin and propagate a split. It is thus wise not to leave water or high-water-content slip on any surfaces experiencing tensile stress during forming.

Context: Splitting at the Plastic Stage

Monday 15th November 2021

Wedging manganese speckle into a cone 6 buff stoneware

Wet clay + manganese = speckled mug

This is actually quite easy to do: Just wedge the clay over the manganese spread out on the board, when the board is clean turn the slug sideways and cut and layer about 20 times (to get 1 million layers). Then wedge normally. Only 0.2% manganese is needed (as a percentage of the dry clay). Since pugged clay contains 20% water it is easy to calculate the dry weight of this piece. For example, suppose this weighs 2 kg: 80% of that is 1.6 kg or 1600g. 0.2% of 1600 is 3.2 grams. Shown is the kind of mug I get. The outside glaze is G2934Y silky matte (opacified with tin) and inside glaze is G2926BW glossy white. It was fired at cone 6 using the PLC6DS schedule.

Context: Manganese Granular, An ordinary white mug: More difficult to make than you think!

Thursday 11th November 2021

Step-by-step how to test and evaluate a new native clay

Two lumps of raw clay dug straight out of a hill

"It Starts With a Lump of Clay", a step-by-step tutorial (from its help system and the link below) on how to document every step (in an account at at of analyzing a raw sample of clay. You will learn things about drying shrinkage, drying performance, particle size distribution, plasticity, firing shrinkage, fired porosity, fired color, soluble salt content, fired strength, etc. We will not just observe these properties, but measure them. In doing so we will characterize the material. We will answer simple questions about how the material forms, dries and fires across a range of temperatures. In doing the testing I will be generating a lot of data. No single factor is more intimidating to new technicians than what do to with this data, how to store it, where to store it, how it can be searched, learned from, compared. This tutorial will erase that question.

Context: How to Find and Test Your Own Native Clays, It Starts With a Lump of Clay: How to Assess a Native Clay, Native Clay, Clay body

Wednesday 10th November 2021

How to make a zero-fired-shrinkage clay

Eight fired clay test bars

This is Plainsman BGP, a terra cotta, mixed with 30% dolomite. Note the "DSHR" column in the SHAB test data (third last column): The drying shrinkage still averages over 7% even with the 30% dolomite, so BGP is very plastic. Notice the "FSHR" (fired shrinkage) column, it is negative for the first five test bars fired at cone 05-01, that means the bars grew in size! But notice the shrinkage hits 0% at cone 1 (bar #6). By cone 2 the trend has reversed to 0.3% shrinkage. The #6 bar is appears to be vitrifying, the color is darkening and it is strong. But notice the last "ABS" column (water absorption), it is 18.7%! This body was intended as a high-porosity ceramic at the lower ranges (it has 25% porosity at cone 05), but the dolomite is also slowing the densification as it goes through the vitrification process. Without the dolomite the top bar would be melting! By cone 1 its firing shrinkage would be 7% and the porosity would be zero. Is this technique practical? Yup! The entire monoporosa wall tile industry is based on it!

Context: Dolomite, Monoporosa or Single Fired Wall Tiles, Firing Shrinkage

Wednesday 10th November 2021

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