Click here for information about DIGITALFIRE Corporation

The first of 15 "Fool-Proof Recipes" wrecked my kiln shelf!

A melt flow tester showing how a normal glazes runs when melting compared to this one

This is recommended in the booklet "15 Tried and True Cone 6 Glaze Recipes". This melt flow tester compares it with a typical cone 6 glossy, G2926B. This recipe is 90% Frit 3110 and 10% kaolin and their booklet recommends adding stains to it. But anyone knowing a little about this frit knows it would run off this flow tester even before bisque temperatures. It is crazy to recommend this. Even as a crackle. For cone 6 it needs to be diluted much more, not just with kaolin but also silica. I knew this would run but I underestimated its melt fluidity. I put a large tile below the tester to catch overrun, yet the melt ran off that and a big three-cm-wide blob melted through the kiln wash and so far into my zircon shelf I cannot chip it off! I cannot imagine how many people have tried this on vertical surfaces and had the same thing happen. The lesson: Use common sense when looking at recipes. Even if their authors did not!

Context: Ferro Frit 3110, Trafficking in Glaze Recipes, Tried and True recipes. Really?, Trafficked online recipes waiting for a victim to try them!, Melt Fluidity, Limit Recipe

Friday 18th September 2020

Over deflocculated vs. under deflocculated ceramic slurry

The slip on the right has way too much Darvan deflocculant. Because the new recipe substitutes a large-particle kaolin for the original fine-particled material, it only requires about half the amount of Darvan. Underestimating that fact, I put in three-quarters of the amount. The over-deflocculated slurry cast too thin, is not releasing from the mold (therefore cracking) and the surface is dusty and grainy even though the clay is still very damp. On my second attempt I under-supplied the Darvan. That slurry gelled, did not drain well at all and it cast too thick. On the third attempt I hit the jackpot! Not only does it have 1.8 specific gravity (SG), but the slurry flowed really well, cast quickly, drained perfectly and the piece released from the mold in five minutes. Interestingly, on a fourth mix I made an error, putting in too much water, getting 1.6SG. The casting behavior was similar to the over-deflocculated slip (even though the Darvan content was much lower). A good casting slip is a combination of a good recipe, the right SG and the correct level of deflocculation.

Context: Slip Casting, Deflocculation, Casting Slip is Not Working

Thursday 17th September 2020

How plastic is a pure kaolin? Could one use it pure for pottery?

A small bowl thrown from pure kaolin

Pure kaolins are clay. It seems logical that "pure clay" is plastic. However most kaolins are not plastic (compared to a typical clay for throwing or modelling). This is because they have a comparatively large particle size (compared to ball clays, bentonites, etc). This small bowl was thrown from #6 Tile kaolin. It is, by far, the most plastic kaolin available in North America. It's throwing properties are so good that one might be misled into thinking it should be possible to make pottery from it. Unfortunately, if it was survive drying without cracks, it would not make it through firing without this happening. This was fired, unglazed, to cone 6. Pure kaolin particles are flat and the throwing process lines them up concentric to centre. So shrinkage is greater across than along them. A filler is needed to separate the kaolin particles. All pure kaolins are also refractory, so even if this bowl had not cracked, the porosity of this piece is very high, completely impractical for functional ware (it needs a flux like feldspar to develop fired maturity).

Context: Kaolin, Plasticity

Tuesday 15th September 2020

The stockpile of St. Rose Red fireclay at the Plainsman plant

A pile of red clay in the lump form

This is the lump form of Plainsman St. Rose Red clay, as received from the quarry. The bright red color is natural iron oxide. That iron cannot be washed out, it is part of the clay crystal structure. The pigmentation is heavy enough that even when this material is a minor part of a body recipe, that body will still fire to a dark color. The color and high cost of St. Rose Red means that it is always heavily diluted with other clays, many of which are highly plastic. Because St. Rose is non-plastic it is a perfect complement to these. It is also refractory, it melts at a much higher temperature than typical clays. It is mined near St. Rose, Manitoba.

Context: Saint Rose Red, Saint Rose Red being delivered. Look what it does at cone 10R!

Tuesday 15th September 2020

Bubbles in Terra Cotta transparent glazes. What to do?

Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers, I ended up getting it on pretty thick. Left: G1916S, a mix of Ferro frits, nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly. Bubbles are of course generated by the body during firing. But also in the glaze. Raw kaolin loses 12% of its weight on firing, that produces gas. Low temperature glazes melt early, while gassing may still be happening. Keeping raw clay content in a glaze as low as possible is good, but at least 15% is normally needed for working properties. Improvements? Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.

Context: Kaolin, Glaze Bubbles, Transparent Glazes

Monday 14th September 2020

Melting glaze balls at various temperatures to see when all carbon has been expelled

G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and clay) fired across the range of 1650 to 2000F (these were 10 gram GBMF test balls that melted and flattened as they fired). Notice how they soften over a wide range, starting below cone 010 (1700F)! At the early stages carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon - think of what a raw glaze might have! Of course, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (in this case 1450F), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible. Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue.

Context: G1916Q, Glaze Melt Fluidity - Ball Test, Ferro Frit 3110, Ferro Frit 3195, LOI, Glaze Bubbles, Transparent Glazes, Soaking, Clouding in Transparent Glazes

Monday 14th September 2020

The perfect storm to create boron-blue clouding at low fire

Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals grow best when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are a lot lower, KNaO fluxing is significantly higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).

Context: Ulexite, Boron Blue, Crystallization

Monday 14th September 2020

Polar Ice Porcelain with Body Stains - by Robert Barritz

Buttons of fired porcelain of many colors

Robert has done really valuable work in this research, what an amazing range of color! I am so grateful he shared this with the rest of us. Surfaces are unpolished and unglazed. All are fired to cone 6. Browns are missing, they can be made using iron oxide. For blacks, Mason 6600 is also effective. The blues require lower percentages than shown here, as low as 2% can be effective. Likewise with others, there is an optimal amount for each stain, beyond that, with increases in percentage the color intensity increase will drop significantly. There is another reason to keep stain percentages to a minimum: To reduce the impact on body maturity (and firing shrinkage). Blues, for example, can significantly heighten the degree of vitrification, even melting the porcelain. If you plan to marble different colors, keeping stain percentage as low as possible is even more important, unless you can do fired shrinkage compatibility testing, for example, the EBCT test. Need to develop your own white porcelain? See the link below.

Context: Mason 6027 Stain, Stain 6200, Mason 6255 Stain, Mason 6266 Stain, Frit VO 6200, Frit VO 6255, Mason 6363 Stain, Mason 6304 Violet Chrome Tin, Mason 6308 Stain, Mason 6306 Stain, Mason 6368 Copen Blue Stain, Stain 6388, Stain 6234, Stain 6219, Mason 6021 Red Stain, Spectrum 2276 Yellow Stain, Mason 6464 Yellow Stain, Mason 6485 Yellow Stain, Mason 6537 Grey Stain, Mason 6386 Blue Stain, Mason 6600 Black Stain, Stain 6121, Stain 6201, Stained Plainsman Polar Ice Porcelain - With Polishing (no glaze), Develop and maintain your own cone 6 porcelain recipe, Ceramic Stain

Monday 14th September 2020

A frit softens over a wide temperature range

Melt flow tests showing the a frit melting from 1550-1750F, Gerstley Borate from 1600-1625F.

This is unlike some raw materials which often melt suddenly. These melt fluidity tests compare the flow of a boron frit across 200 degrees F. It first starts flowing at 1550F (although it began to turn to a glass at 1500F) and is running off the bottom of the runway by 1750F. The Gerstley Borate, on the other hand, goes from no melting at 1600F to flooding off the bottom by 1650F!

Context: B2O3, Frit Fusibility Test, The Chemistry, Physics and Manufacturing of Glaze Frits, Ferro Frit 3195, Gerstley Borate, Melting Temperature, Frit, Common frits begin melting

Saturday 12th September 2020

Melt fluidity comparison of frits - 1650F

Fired at 350F/hr to 1650F and held for 15 minutes. FZ16 has turned crystal clear and spread out across the runway (has low surface tension). Frit 3110 has so much surface tension that the flow can be lifted off the tester. Since 1600F Gerstley Borate has gone from unmelted to passing all the rest!

Context: Frit B325, Frit LA-300, Ferro Frit 3249, Ferro Frit 3195, Ferro Frit 3134, Ferro Frit 3124, Ferro Frit 3110, Fusion Frit F-524, Fusion Frit FZ-16, Fusion Frit F-75, Fusion Frit F-69, Fusion Frit F-38, Fusion Frit F-15, Ferro Frit 3602, Melt fluidity comparison - 1600F, Comparing the Melt Fluidity of 16 Frits

Saturday 12th September 2020

Contact Us:

Use the contact pages within or outside your account at Insight-Live.com


Test, Document, Learn, Repeat in your account at insight-live.com

Login to your online account

Chemistry plus physics. The on-line successor to desktop Insight. Get an account for as little as $15. It does so much more.

Maintain your recipes, test results, firing schedules, pictures, materials, projects, etc. Organizing that data will better empower you to adjust, formulate and fix your recipes and processes. Link, share and publish.

Conquer the Glaze Dragon With Digitalfire Reference info and software

Still available for Mac, PC, Linux

Interactive glaze chemistry calculations. Free and unrestricted (legacy, no longer in development).


What people have said about Digitalfire

  • This site is very informative. If the average layperson were to read this site, they would be blown away, and in my case, inspired to learn more. I like to give credit where credit is due, and this site rocks.
  • I'm trying to access a great article on deflocculation. I have formulated so many casting slips over the years with the help of what I learned in that article and on this site.
  • Your glaze expertise is the piece of the clay puzzle that I need to relax and enjoy this artistic journey.
  • I think Glaze chemistry is an extremely under-appreciated and under-estimated field of science.
  • I have a background in Geology (BS-1973-New Mexico Tech), and with a few other degrees here and there - and have been doing quite a bit of Pythoning, and hanging-around my wife who does (I think (but then, of COURSE I am biased)) terrific pots... I am always interested in what she does with glazes (especially when I see the mineral-names from my Geology days on the bins in her studio... BUT - as an "engineer", sometimes I want 'more' knowledge - and I find your web-site VERY VERY good at that, it tells me a lot of things - gives me 'value added' and 'information' that I haven't found elsewhere! Especially when I think about 'geo-chem'.
  • All your info on glaze chemistry and material characteristics are very informative and great catalysts for pushing me to take things a bit further. Most appreciated.
  • However I do believe you have the most comprehensive information anywhere in the world, and I would not hesitate to contact you for any information I required for Ceramic Glaze. I will also refer my associates to you if ever they require assistance.

What people have said about Insight-Live

  • I keep telling people that I just found the best ceramic website on the internet. Paid for an account on insight live.
  • I'm accepting the challenge. First: find/develop a good base recipe. One for clear and one for celadon. Then add the colors. And play with "surface." Experiment. Keep impeccable notes. Change one variable at a time. Yada, yada.
  • I have longed admired your website, and everything you've contributed to better understanding ceramic technology.
  • And I want to add - you are doing a magnificent job with your webpage. Just unbelievable amount of work and information you put there. I have learned a lot from your webpage. Thank you.
  • Thank you again for the amazing website and wealth of information!!! InsightLive is proving to be an incredible asset as well!!!
  • Hello Mr. Hansen. Your digitalfire.com site is AMAZING! I use it considerably. Thank you so much for all the information you offer! I tell all my students and friends in clay to come here to learn.
  • I'm a professional potter from Western Australia with a 30 year career - self taught. I came across Digitalfire when researching raw materials - an incredible resource you've put together!

DigitalFire Logo

Privacy Policyhi11