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If your functional ware crazes you absolutely must fix it

If your functional ware crazes you absolutely must fix it

This glaze and body and not compatible, the body thermal expansion is too low and the glaze it too high. It not only creates a germ zoo but severely weakens the piece. I tapped this lightly with a spoon about 1-2 cm below the rim, doing so repeatedly. Right from the start the piece had no ring, no strength. With each tap the pitch dropped. The piece is breaking apart the same way auto-glass does in a collision. This type of functional ware is entirely unacceptable. If you use commercial glazes test their compatibility with the bodies you use. If you make your glazes, adjust their recipes to reduce the thermal expansion until no crazing occurs using a thermal stress test (e.g. the IWCT test, which anyone can do).

Tuesday 9th October 2018

The shrinkage of a porcelain hand-made tile

The shrinkage of a porcelain hand-made tile

Can you help with some math? Dry shrinkage is obvious. Fired fired shrinkage is not the total but the shrinkage from dry to fired. And you cannot just add the dry and fired numbers together to get the total because the fired shrinkage value is based on the dry length, not the original. In this example, 6.25 dry shrinkage plus 6.66 fired equals 12.9 whereas the actual total shrinkage is 12.5. But what if you need to calculate the actual total shrinkage if you only have drying and fired shrinkage? I am having trouble deriving the formula. Anyone can help?

Tuesday 9th October 2018

Measuring clay test bars done by Luke Lindoe 40 years ago

Measuring clay test bars done by Luke Lindoe 40 years ago

Luke Lindoe prospected Montana and Idaho for clays during the 1970s. He found an amazing variety of fireclays, earthenwares and stonewares. Every color, texture, plasticity. For each he made test bars to fire at different temperatures. Our M2 and Troy clays originated from this work. We just found these bars, but do not have Luke's shrinkage and porosity data, so are measuring them now. He code-numbered each and stamped them with four-inch marks. So we can derive the total fired shrinkages and measure the porosities. We can tell a lot about the plasticity of each by the nature of the cut lines. The texture also is obvious. Now we just need to start searching Luke's map archives to find out where all of these are.

Wednesday 3rd October 2018

We have to fight with the fibreglass industry to get kaolin!

We have to fight with the fibreglass industry to get kaolin!

These are bags from three recent truckloads of 880 bags each. Order-delivery delays are getting longer and longer as the fibreglass industry is making more and more demands on kaolin suppliers. This means we have to store this material in larger quantities and for longer periods than in the past. And we must be more diligent in testing for consistency because manufacturers are catering to fibreglass instead of ceramics. When this is coupled with the decline of ceramic manufacturing in North America it means maintaining and documenting the properties important to ceramics are becoming less important to kaolin manufacturers.

Wednesday 3rd October 2018

Here is what it takes to make sure P700 has minimal fired specks

Here is what it takes to make sure P700 has minimal fired specks

We had to sample every pallet of a 1500 bag bentonite shipment. On testing each one we found dark-coloured particulates. Then we determined which pallets where the worst and did a second round of testing. Then we mixed up 5000 gram P700 test batches from three pallets, made ware and tiles, clear glazed and fired it all to cone 10R (with heavy reduction). We also prepared samples and returned them to the manufacturer for further testing in their lab. As it turned out, the dark particles were not iron-containing and we found only a few tiny specks.

Friday 21st September 2018

Three visual glaze mechanisms make this piece unique

Three visual glaze mechanisms make this piece unique

"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954J black engobe on the inside and part way down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).

Friday 21st September 2018

The matte version covers better, looks better

The matte version covers better, looks better

This is G2934 matte (cone 6) and G2931K glossy base recipe having the same Cerdec yellow encapsulated stain been added (about 10%). You really need to see these side-by-side to appreciate how much richer the silky matte version looks (and feels). AMACO has popularized the bright glossy transparent look in its 'Celadon' product line (while the effect looks good on smaller areas, for cover glazing it appears 'washed out'). An additional factor to consider is that encapsulated stains begin to decompose around cone 6, this can produce orange-peel and blisters on glossy glazes, but with this matte base we have not seen it happen with any color. Google 'G2934 recipe' to learn more, it is being talked about everywhere, a truly amazing base.

Friday 21st September 2018

A mug cracks before your eyes because of glaze compression

A highly vitreous, thin walled mug is glazed inside-only. The glaze has a thermal expansion that is too low and it is under compression, pressing outward. A tap with a spoon is enough to trigger a sudden crack. It opens under the pressure. Had it been glazed on the outside also it would likely survive, but this test still indicates that it would be better to raise the expansion a little.

Friday 21st September 2018

Program your firings manually, calibrate the final temperature

Program your firings manually, calibrate the final temperature

Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). We need it to actually go to cone 10, the only way to do that is verify with a cone (self supporting cones are the only accurate way). Then make a note in the record for that schedule in your account at insight-live.com.

Thursday 30th August 2018

Polar Ice at cone 10 reduction. Stunning!

Polar Ice at cone 10 reduction. Stunning!

The Polar Ice data sheet has been changed. In the past we have been hesitant about firing it over cone 6. This is because it is just so vitreous and translucent that firing it higher seemed to be asking for trouble (like warping, blistering). But at cone 10R it is still resistant to warping. And fires this beautiful blue-white. The translucency, amazingly, is about the same: Incredible! This is among the most incredible pieces we have ever made in the studio here, it is hard to believe it is possible produce this kind of quality in a pottery studio.

Tuesday 28th August 2018

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