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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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Why does the glaze on the right crawl?

This is G2415J Alberta Slip glaze on porcelain at cone 6. Why did the one on the right crawl? Left: thinnest application. Middle: thicker. Right thicker yet and crawling. All of these use a 50:50 calcine:raw mix of Alberta Slip in the recipe. While that appears fine for the two on the left, more calcine is needed to reduce shrinkage for the glaze on the right (perhaps 60:40 calcine:raw). This is a good demonstration of the need to adjust raw clay content for any glaze that tends to crack on drying. Albertaslip.com and Ravenscrag.com both have pages about how to calcine and calculate how much to use to tune the recipe to be perfect.

Thursday 1st August 2013

Why does the right glaze crawl and the left does not?

The glaze on the right is crawling at the inside corner. But two factors contribute. First, the angle between the wall and base is sharper than on the left, and a thicker layer of glaze has collected there (the thicker it is the more likely it is to crack on drying). In addition, the glaze on the right also shrinks more because it has a higher clay content.

Wednesday 25th July 2012

Cone 6 kaolin porcelain verses ball clay porcelain.

Typical porcelains are made using clay (for workability), feldspar (for fired maturity) and silica (for structural integrity and glaze fit). These cone 6 test bars demonstrate the fired color difference between using kaolin (top) and ball clay (bottom). The top one employs #6 Tile super plastic kaolin, but even with this it still needs a 3% bentonite addition for plasticity. The bottom one uses Old Hickory #5 and M23, these are very clean ball clays but still nowhere near the whiteness of kaolins. Plus, 1% bentonite was still needed to get adequate plasticity for throwing. Which is better? For workability and drying, the bottom one is much better. For fired appearance, the top one.

Friday 6th March 2015

Underfiring a clay is OK if the glaze fits? No it is not.

Left: Plainsman M340 fired to cone 6 where it achieves about 1.5% porosity, good density and strength. Right: H550, a Plainsman body intended to mature at cone 10, but fired to cone 6 using the same glaze. Although the glaze melts well and the mug appears OK, it is not. It is porous and weak. In fact, it has cracked during use (the crack runs diagonally down from the rim). It was then dipped into water for a few moments and immediately the water penetrated the crack and began to soak into the body (you can see it spreading out from the crack). If this glaze were to craze the entire thing would be waterlogged in minutes.

Friday 10th April 2015

Can you make things from zircopax? Yes.

Only 3% Veegum will plasticize Zircopax (zirconium silicate) enough that you can form anything you want. It is even more responsive to plasticizers than calcined alumina is and it dries very dense and shrinkage is quite low. Zircon is very refractory (has a very high melting temperature) and has low thermal expansion, so it is useful for making many things. Of course you will have to have a kiln capable of much higher temperatures than are typical for pottery or porcelain to sinter it well.

Thursday 16th April 2015

Should I glaze the outside of this mug now? Why no?

This bisque mug has been glazed on the inside. But the bisque has absorbed water from that glaze and this thin walled mug is now water logged as a result (except at the thicker base). It does not have the absorbency needed to build up a thick enough layer of glaze on the outside. Even if it did, the water from the two glazes would wet the bisque so much that its drying time would be greatly extended. This is a problem because the mechanism of attachment of glaze to body is fragile and works best when the glazes dries quickly (if drying is too slow bubbling and cracking can result).

Friday 17th April 2015

Water-logging happens when a clay is underfired

The cone 6 glaze is well developed, it is not crazed. But the clay underneath is not developed, not vitreous. This crack happened when the mug was bumped (because of poor strength). It is barely visible. When the mug is filled with water, this happens. How fast? This picture was taken about 5 seconds later. If this was crazing, and this piece was in actual use, the clay would gradually become completely water logged. Then one day someone would put it in the microwave! Boom.

Thursday 16th April 2015

How to create a wood ash glaze

This Wood ash glazed cup has been fired at cone 6. It was the product of a development project, a series test recipes having the objective of maximizing the percentage of ash. Obviously, it contains a little iron to stain it brown, this brings out the variegation better. Ash generally contains low percentages of Al2O3, a critical oxide needed for stable glass development. I added kaolin (about 20%) to suspend the slurry (it supplies Al2O3 also). Ashes contain lots of fluxing oxides, but they still may need a little help to melt a glaze at cone 6, I added feldspar (it also supplies needed Al2O3). If that is not enough, I add a little gerstley borate or a borax frit like Ferro 3124. If crazing occurs silica is needed. In the end I got a recipe with about 50% ash.

Wednesday 28th November 2012

Chrome tin pinks are easier using a stain than chrome and tin

The Ravenscrag Slip based burgundy glaze on the outside of these mugs is made by fluxing Ravenscrag with 20% Ferro Frit 3134 and adding 10% Mason 6006 burgundy stain (actually these have a little less stain, about 8%). This stain works better than using raw chrome and tin. This glaze functions very well on porcelains and breaks white on the edges to highlight contours.

Thursday 16th April 2015

Simple dilatometric curve produced by a dilatometer

Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.

Thursday 19th February 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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