Calculate the total shrinkage of a porcelain hand-made tile
Plainsman Clays publish dry and fired shrinkage data for their clay bodies. Dry shrinkage is, of course, the shrinkage from wet to dry. Fired shrinkage is not, however, the total from wet to fired. Rather it is 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). It is not a huge difference but this is the way to calculate it correctly if you only have drying and fired shrinkage. Thanks to Tom Hittie for deriving this for us.
Context: Firing Shrinkage, Drying Shrinkage
Tuesday 20th November 2018
Using commercial glazes? You still need to know about specific gravity.
The glaze in this jar was 'goop', impossible to paint on. I did not know whether I needed to add water or try to
deflocculate it (although the former is more likely and in keeping with what Laguna says on its website). I measured the
specific gravity, it was 1.7, so clearly it needed water. It took 125cc to bring the
specific gravity down to 1.5. However, it was still thick and dried immediately after painting on, clearly it does not contain enough gum for brushing. The next time I will add a mix of 50:50 gum solution and water for better paintability. The bright side: I got considerably more than a pint after adding the water, a big difference from some other commercial glazes which are mostly water.
Context: Brushing Glazes, Specific gravity
Friday 16th November 2018
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!
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.
Context: Kaolin
Wednesday 3rd October 2018
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.
Context: Bentonite, Plainsman P700
Friday 21st September 2018
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).
Context: Mechanism
Friday 21st September 2018
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.
Context: Glaze Compression
Friday 21st September 2018
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.
Context: Controller, Cones, Firing Schedule
Thursday 30th August 2018
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.
Context: Plainsman Polar Ice cone 6 translucent porcelain
Tuesday 28th August 2018