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Tony Hansen's Thousand-Post TimeLine

I am the creator of Digitalfire Insight, Digitalfire.com and Insight-live.com. I have made hundreds of posts like these on my Facebook page and personal timeline. My posts are like no others, they help you understand your glazes and clay bodies, take control. They are also part of the Digitalfire Reference Database (referenced from one or more articles, glossary entries, materials, oxides, test procedures, etc). Visit and Like my page to get a notification each time I post.

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Plainsman iron red clays with rutile blue Alberta Slip glaze

Cone 6 mugs made from Plainsman M350 (left) and M390 dark burning cone 6 bodies. The outside glaze is Alberta-Slip-based GA6-C rutile blue and the inside is GA6-A base (20% frit 3134 and 80% Alberta Slip). That inside glaze is normally glossy, but crystallizes to a stunning silky matte when fired using the schedule needed for the rutile blue (cool 100F and soak, slow cool to 1400F).

Friday 27th March 2015

Specific gravity of a glaze using a scale and measuring cup

The specific gravity of a glaze slurry is simply its weight compared to water. Different glazes optimize to different specific gravities, but 1.4 to 1.5 is typical (highly fritted glaze are higher). To measure, counter-weigh a plastic measuring cup on your scale and fill it with 500 grams of water and note how high the water fills it (hopefully to the 500cc mark!). Fill the container with your glaze to the same place. Divide its weight by the number of ccs (in this case, 500) and you have the specific gravity. The more you weigh the more accurate is the test.

Wednesday 25th March 2015

Measuring specific gravity

This is the easiest way to measure the specific gravity of a glaze if it is not in a container deep enough to float a hydrometer (or if it is too thick to float it properly). Just fill to the 100cc mark and the scale reads the specific gravity. Be careful on cheap plastic graduated cylinders like this, check them with water and correct the true 100cc mark if needed. You could actually use any tall narrow container you have (if you make the 100cc level).

Wednesday 16th July 2014

The feldspar can make a big difference in porcelain whiteness

The only difference between these two cone 6 porcelains is the feldspar. On the left is Minspar 200, on the right is Nepheline Syenite. They contain 309%.

Monday 23rd March 2015

Cleanest kaolin porcelain vs. ball-clay-only porcelain!

These cone 6 clear-glazed porcelains demonstrate just how white you can make a porcelain if you use white burning kaolins and bentonites instead of ball clays. Both contain about 40% clay. The one on the left employs New Zealand kaolin and Veegum plasticizer, the one on the right Kentucky ball clays (among the whitest of ball clays in North America) and standard bentonite. Both are zero porosity. The glaze surface is a little more flawless on the right one (possibly because ball clays have a lower LOI than kaolins).

Tuesday 17th March 2015

Brown soluble salts that appear after drying, but disappear on firing

The soluble salts dissolved in the water of plasticity of this red body have migrated through the white engobe during drying of these earthenware cups. The cups were upside down so all the solubles have been left on the outside surface. The red body is made using a high percentage of Redart clay (a widely available commercial low-fire low-plastic clay in North America). It is plasticized using added ball clay. The brownish material is organic, because after bisque firing it has disappeared.

Thursday 26th February 2015

It shrinks much more yet cracks less. How is that possible?

Two mugs have dried. The clay on the left shrinks 7.5% on drying, the one on the right only 6%. Yet it consistently cracks less! Not the slightest hairline crack, not even at the handle joins. Why? Green or dry strength. If the dry clay matrix has the strength it can resist cracking even if there are stresses from uneven drying. The clay on the right is made using Kentucky ball clay, which has good plasticity but fairly low drying strength. The clay on the left is a native terra cotta, very plastic and very strong in the green state (likely double or triple the white clay). To demonstrate further: If paper fiber were added to the white clay, it would not crack. Why? Not because it would shrink less with the added fiber, no, the shrinkage would stay the same. Increased strength imparted by the fiber would give it the power to resist cracking.

Tuesday 1st January 2013

Powder to slurry to dewatered to mugs with handles in 1 hour. How?

These are made from a whiteware having 0.5% paper fiber added. The paper addition speeds up the dewatering time. The clay throws well and the handles pulled extremely well. The mugs were stable enough to attach the handles immediately. Then I just left them out to dry overnight. There were no cracks in the morning, so I turned them upside down and rewet the bottoms by setting a wet sponge on them for a few minutes. That enabled me to trim them.

Monday 16th March 2015

Paper clay is easy to make. And really different!

This measuring cup contains 30 squares of toilet paper or 11 grams (which has disintegrated quickly and has been propeller-mixed). I am about to dump the paper fiber and 1000 grams of plastic porcelain powder into the water and then mix that up and pour the slurry onto a plaster bat. Although the fiber is only 1% by weight of the dry mix, this completely changes the working properties of the clay. It is still plastic, but much more difficult to cut with a knife or wire. It rolls out nicely into very thin slabs and they are very tough and easy to manipulate and build with. As it hardens it is still pretty plastic.When forced to bend it slowly breaks as the fibers release across the boundaries. Two dry pieces of this clay can be joined using only water and they stick together! Of course this paper needs to burn out during firing, so you need good ventilation on your kiln. You might think that this paper clay shrinks much less than the non-paper version. Actually, it shrinks more (likely because of the increased percentage of water needed). The paper is imparting strength, that strength is enough to resist cracking on drying.

Friday 13th March 2015

Various grogs available in North America

Examples of various sized grogs from CE Minerals, Christy Minerals, Plainsman Clays. Grogs are added to clays, especially those used for sculpture, hand building and industrial products like brick and pipe (to improve drying properties). The grog reduces the drying shrinkage and individual particles terminate micro-cracks as they develop (larger grogs are more effective at the latter, smaller at the former). Grogs having a narrower range of particle sizes (vs. ones with a wide range of sizes) are often the most effective additions. Grogs having a thermal expansion close to that of the fired body, a low porosity, lighter color and minimal iron contamination are the most sought after (and the most expensive).

Thursday 12th November 2009


These posts are actually pictures referenced on pages in The Digitalfire Reference Database, thousands of pages of explaining things you need to know to formulate, adjust and troubleshoot traditional ceramic bodies and glazes. It is organized as: Oxides, minerals, materials, recipes, articles, glossary, hazards, library, MDTs for INSIGHT, pictures, properties, firing schedules, significant temperatures, tests and troubleshooting. Level 2 desktop INSIGHT and Insight-Live both interact with it.

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