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Conquer the Glaze Dragon With Digitalfire INSIGHT Glaze Chemistry Software

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Focused on ceramic glaze chemistry calculations.

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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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Lignite contamination in manufactured porcelain bodies

These tiny black specks on the rim of a dry mug are from lignite contamination in the ball clay (I have highlighted them by wetting the surface a little). They can cause pinholing in glazes. Ball clays are air floated (after grinding, a stream of air takes away the lighter particles and the heavier ones recycle for regrinding). These particles are lighter and more difficult to separate. Ores in Tennessee are higher in coal than those in Kentucky. American and Canadian clay body manufacturers who confront ball clay suppliers with this contamination find that ceramic applications have become a very small part of the total ball clay market, complaints are not taken with the same seriousness as in the past.

Thursday 5th March 2015

Pinholing, a secret to solving it

Same glazes, same clay, yet the right one is pinholing badly. Why? These are thick pieces, they need time for heat to penetrate. Right: Fired to cone 6 (2190F) and soaked 15 minutes. Left: Fired to cone 6, soaked 15 minutes, then cooled 100 degrees and soaked 45 minutes. Pinholes and dimples are gone, the clay is also more mature, and the glaze is glossier and melted better. Why does this work? Likely because whatever is gassing and creating the pinholes will have stopped by 2090F. Also, these are boron-fluxed glazes, they likely stay fluid all the way down to 1900F (so I could drop even further before soaking).

Monday 27th May 2013

Will this crawl when fired? For sure!

This dry glaze is shrinking too much, it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe. Clay is needed to suspend the other particles (they would quickly settle to the bottom of the bucket without it), but too much causes excessive shrinkage. Fixing this problem is not nearly as difficult as most people think. You can reduce shrinkage by calcining part of the clay or swapping a clay component for another of similar chemistry but lower shrinkage. The best way: Use glaze chemistry to source some Al2O3 (contributed by the clay) from feldspar instead. Of course this involves juggling amounts of other materials in the recipe to maintain the overall chemistry.

Wednesday 25th February 2015

Lab testing a clay for its physical properties

SHAB test bars, an LDW water content sample and a DFAC drying disk about to be put into a drier. The SHAB (shrinkage-absorption) bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI from these measured values.

Thursday 26th February 2015

Making your own hexagonal shelves from alumina

The home-made kiln shelf (left) was fired it at cone 10. It is half the weight (and thickness) of the cordierite one (but remember that it does not have the thermal shock resistance of cordierite). It is made from a body consisting of 96.25% calcined alumina and 3.75% Veegum. It rolls out nicely and dries perfectly flat over about three days. But the Veegum does not give up its water easily. I cut it 1/4" larger than the other and it has fired to the same size; this body has incredibly low shrinkage.

Tuesday 3rd September 2013

What if you just cannot solve a pinhole problem?

Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in production either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers.

Tuesday 27th January 2015

Cross section is an important factor in avoiding cracks

The cross section of a bowl. For the best success in drying and firing, it is advantageous to have as even of a thickness as possible. But it is also important not to have sharp concave angles. It would have been possible to make the section outside the foot ring thinner by creating a more abrupt concave contour, but that contour, if too sharp, could offer a point of weakness where a crack could start.

Monday 26th January 2015

Fixing a crawling problem with Ravenscrag Tenmoku

Crawling of a cone 10R Ravenscrag iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. Ravenscrag.com has information on how to do this.

Tuesday 3rd February 2015

A Redart cone 03 body shines when it come to ease of glaze fit

These bowls are fired at cone 03. They are made from 80 Redart, 20 Ball clay. The glazes are (left to right) G1916J (Frit 3195 85, EPK 15), G191Q (Frit 3195 65, Frit 3110 20, EPK 15) and G1916T (Frit 3195 65, Frit 3249 20, EPK 15). The latter is the most transparent and brilliant, even though that frit has high MgO. The center one has a higher expansion (because of the Frit 3110) and the right one a lower expansion (because of the Frit 3249). Yet all of them survived a 300F to icewater test without crazing. This is a testament to the utility of Redart at low temperatures. A white body done at the same time crazed the left two.

Sunday 1st February 2015

A frit manufacturer that freely publishes chemistry? Yes!

I am impressed with this Italian frit company, Reimbold & Strick. They publish the chemistry of tons of their frits. It is quite educational, by studying the chemistry of their matte frits, for example, you can see the various mechanisms that produce the effect.

Sunday 1st February 2015


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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