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Tony Hansen's Thousand-Post TimeLine

I am the creator of Digitalfire Insight, the Digitalfire Reference Database and Insight-live.com. ... more

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Should you use a porcelain as an engobe on stoneware?

Should you use a porcelain as an engobe on stoneware?

This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. But during firing the porcelain has a much higher fired shrinkage and it pulls the bar toward itself. ... more

Monday 21st May 2018

Terra cotta and a surprising thing about thermal shock

Terra cotta and a surprising thing about thermal shock

This terra cotta cup is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 25 seconds under direct flame against the sidewall before a crack occurs. Typical porcelains and stonewares would survive 10 seconds. Super vitreous porcelains 5 seconds. This is an advantage of ... more

Monday 21st May 2018

Flame thermal shock failure test

Flame thermal shock failure test

A 3 inch by 3/16 thick tile is flamed and timed until it cracks. The crack usually is audible and sometimes spectacular (the tile flying apart). However if the body is heavily grogged the crack can grow slowly (often from the edge inward). We log the result using the TSFL test.

Monday 21st May 2018

A root-of-two series of test sieves

A root-of-two series of test sieves

The coarsest screen is at the top, the finest on the bottom. The opening for each is shown on the label. They are chosen such that each successive screen going down has an opening that is about half the area of the one above it. Using this series you can produce a practical measurement of the ... more

Monday 21st May 2018

High drying shrinkage of Plainsman A2 ball clay (DFAC disk)

High drying shrinkage of Plainsman A2 ball clay (DFAC disk)

This DFAC test disk shows the incredible drying shrinkage that a ball clay can have. Obviously if too much of this is employed in a body recipe one can expect it to put stress on the body during drying. Nevertheless, the dry strength of this material far exceeds that of a kaolin and when used ... more

Monday 21st May 2018

The white one feels smoother, but it is actually far coarser. Why?

The white one feels smoother, but it is actually far coarser. Why?

Large particle kaolin (left) and small-particle ball clay (right) DFAC tests (for drying performance) demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to ... more

Monday 21st May 2018

Grog does not always have the intended effect

Grog does not always have the intended effect

These DFAC test disks (drying performance) show that minor additions of grog do not reduce the fired shrinkage of this medium fire stoneware much. Nor do they improve its drying performance. In this example, a 10% addition has not reduced shrinkage appreciably nor has it improved drying performance. ... more

Monday 21st May 2018

Do grog additions always produce better drying performance?

Do grog additions always produce better drying performance?

This DFAC test for drying performance compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise there appears to be no improvement. While the grog addition reduces drying shrinkage by ... more

Monday 21st May 2018

What happens when engobe fired shrinkage is too high for the body

What happens when engobe fired shrinkage is too high for the body

The EBCT test bars (engobe compatibility) in the foreground demonstrate the issue (they sandwich the engobe and body as a bi-clay strip). After firing at cone 10R they have curled toward the engobe side, indicating that it is shrinking more. On the mug the engobe has done likewise, shrinking more than the body and creating a crack pattern.

Sunday 20th May 2018

Roasting Ravenscrag Slip instead of calcining

Roasting Ravenscrag Slip instead of calcining

This is the Ravenscrag Slip I used to calcine at it 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. I hold it for 2 hours at 1000F to make sure the heat penetrates. It is not actually calcining, since not all crystal water ... more

Sunday 20th May 2018

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