All-New Digitalfire Reference Library!

Technical information from Tony Hansen on traditional ceramics without ads, surveillance, distractions or interruptions. What you find here is about how to improve the quality of ceramics that you make or manufacture, how to adjust and fix issues with materials, recipes and processes. It is about understanding things, right down to the physics and chemistry level if needed. It is also about measuring and quantifying things, and recording that information. Yet what you find here is practical and accessible to anyone doing ceramics. This site is always under-construction and always being adjusted.

Features of the new Digitalfire Reference Library:

This will run in parallel to the old library for now, links there will come here more and more over time. URLs for old pages will forward automatically to new ones when the full change-over is done.

Recent Pictures

Stuck at home with no ceramic supplies? Time to organize!

Are your records in a messy binder? You could be using an account at! Move your recipes first, assigning each a code number. Then, in your studio/lab, label every fired sample, bucket, jar, glaze test, bag with the corresponding code number. Put in pictures for each recipe. Enter your firing schedules. Research the solutions to issues you are facing with glazes at the Digitalfire Reference Library (ask us questions using the contact form on each of the thousands of pages there). Then start planning improvements and tests. Choose a recipe you need to improve/evolve, duplicate it, increment the code number, make changes, enter explanatory notes. With this preparation you will hit the ground running back at work.

Context: The Digitalfire Reference Library by Tony Hansen, Insight-Live

Wednesday 8th April 2020

A gunmetal glaze I have wanted for decades!

After 40+ years of making pottery I finally have a perfect gunmetal black. It has an incredible silky surface. It does not cutlery mark. It does not craze on anything. It is easy to clean. This is G2934Y with 6% Mason 6600 black stain firing using the PLC6DS schedule. I had to tune it a bit, adding about 15% glossy G2926B, because it was a little too matte on initial firings. But now it is perfect. These are heavy mugs made using the M340 casting recipe (and the casting-jiggering process). The speckled mug was made by casting a thin layer of the speckled version of the slip first, then filling the mold with the regular slip. I used a 40-minute cast to get walls nice and thick!

Context: Cooling rate drastically affects the appearance of this glaze, Gunmetal glaze

Saturday 4th April 2020

How is it possible for the same body to work well at both cone 04 and 6!

White cone 04 bodies are not vitreous and strong and neither is this. But it is plastic, smooth and fits common low fire glazes. How? 15% Nepheline Syenite (also 50% Plainsman 3D, 35% ball clay and 3% bentonite). The unmelted nepheline particles impose their higher thermal expansion on the fired ceramic. Spectrum 700 clear glaze does not craze and does not permit the entry of water (the mug is glazed across the bottom and fired on a stilt). The mug on the right is made from the same clay, it has been fired ten cones higher, cone 6! Here the nepheline is acting as a flux, producing a dense and very strong stoneware (with G2926B, GA6-B glazes). This is incredible! One note: This cannot be deflocculated and used for casting, soluble salts in the 3D gel the slurry.

Context: Nepheline Syenite, 3D Clay, Low Fire White Talc Casting Body Recipe

Saturday 4th April 2020

Red burning, customer-found terra cotta clays tested

We tested four different clays (brought in by customers). One is from BC and three from Alberta. These fired sample bars show rich color, low soluble salts and high density and strength at very low temperatures. L4233 (left): Cone 06 to 3 (bottom to top). Reaches stoneware-density at cone 02 (middle bar). Plasticity is very low (drying shrinkage is only 4.5%). But, it is stable even if over-fired. L4254 (center bottom): Cone 04,02,3,4 (bottom to top). Very fine particled but contains an organic that is gassing and bloating the middle two bars. L4243: Fires lighter and looks stable here (cone 02,01,1,2 shown) but melts suddenly less than a cone above the top bar (well before vitrification is reached). L4242 (right): Hyper-plastic, with 12% drying shrinkage! Already melting by cone 02 (third from top). Achieves almost zero porosity (porcelain density) at cone 04 (#2 bar). Even when mixed with 20% kaolin and 20% silica it still hits zero porosity by cone 1. What next? I'll mix L4233 (left) and L4242 (right), that should produce stoneware density at cone 02 (about 1% porosity).

Context: How to Find and Test Your Own Native Clays, Terra cotta, Soluble Salts, Drying Shrinkage, Vitrification, Bloating

Saturday 4th April 2020

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 is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.

Context: GR10-A, GR6-A, Ravenscrag Slip, Sterile white vs. pure Ravenscrag Slip as a liner glaze at cone 10R, Calcination

Saturday 4th April 2020

This glaze can dry on metal without cracking, even though it is thick!

This sample of glaze was dried under a heat lamp to measure its water content. If a glaze that is this thick can crack this little during drying and adhere even to stainless steel there is absolutely no reason you need to suffer glazes cracking during drying on bisque ware. This one is very high in frit with about 15% No. 5 ball clay. Drying cracking problems can be fixed using Digitalfire Insight, it enables you to juggle a recipe to reduce and substitute plastic ingredients while maintaining the chemistry.

Context: G2916F

Saturday 4th April 2020

This clay was layered to 16,000. Yet still not mixed? Why?

Soft and stiff slabs were inter-layered (giving eight layers total). Then the piece repeatedly cut in half and slammed downward to re-flatten. Eleven times (doubling the number of layers each time to get 16,536). Yet it is still not mixed! 30 seconds of wedging is all it took to finish the job. Wedging is a very effective mixing technique. A pugmill would easily mix this also.

Context: Wedging

Saturday 4th April 2020

The same clay in lump and powder form. Which is heavier?

Which one of these samples weighs more, the raw lump form of the clay or after it has been ground into a powder? Wrong. It is the lumps. Even though there is all that empty air space between those lumps, there is even more air spaces in the powder. The top one weighs 1662 grams (there is a 500g counter-weight barely visible), the bottom one is 1255. The finer I grind it the lighter it will be. If I were to fill in all the voids between the lumps on the top one with smaller sized lumps I could get alot more weight yet! It works the same on the ultimate particle level, when we combine powders of varying particle sizes we get a more dense and stronger dried product.

Context: Particle Size Distribution

Saturday 4th April 2020

Measuring slip viscosity the easy way

A Ford Cup being using to measure the viscosity of a casting clip. These are available at paint supply stores. It drains water in 10 seconds. This casting slip has a specific gravity of 1.79 and we target a 40-second drain. Maintenance of viscosity and specific gravity are vital to an efficient process in slip casting.

Context: Low Budget Testing of the Raw and Fired Properties of a Glaze, Understanding the Deflocculation Process in Slip Casting, Viscosity, Slip Casting, Specific gravity, Rheology, Deflocculation

Saturday 4th April 2020

What happens when a limestone clay mix is fired to cone 6?

The top bar is a mix of calcium carbonate and a middle temperature stoneware clay (equal parts). On removal from the kiln it appears and behaves like a normal stoneware clay body, hard and strong. However, pour water on it and something incredible happens: in a couple of minutes it disintegrates (as it rehydrates). And generates lots of heat as it does so.

Context: Calcium Carbonate, Limestone, Calcination

Saturday 4th April 2020

Now that is a translucent porcelain!

These are two cone 6 transparent glazed porcelain mugs with a light bulb inside. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity). Right is a zero-porosity New Zealand kaolin based porcelain called Polar Ice (from also)! The secret to making a plastic porcelain this white and translucent is not just the NZ kaolin, but the use of a very expensive plasticizer, VeeGum T, to enable maximizing the feldspar to get the fired maturity.

Context: Bone Ash, Formulating a Porcelain, New Zealand Halloysite, Translucency, Bone China, A 3-minute Mug with Plainsman Polar Ice

Friday 3rd April 2020

Here's how we used to record test results before

Side-by-side presentation. That’s the best. But I magine if you could put, side by side; the recipes, pictures, notes, data, of any recipe test you had ever done. Even results of testing you did on commercial prepared glazes and glaze combinations. And be able to link, search, print, share them. That’s what you do in Insight-live. Pottery has always been about the data, we just let that information die before! Now we can learn so much more from it. Photo courtesy of Brielle Rovito, Burlington, Vermont, USA.

Context: Code Numbering, Insight-Live

Thursday 2nd April 2020

Metal leaching from ceramic glazes: Lab report example

This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and waste water analysis. They also provide pottery glaze leaching analyses (the water is kept in contact with the glaze for many days then analysed for trace levels of specific metals). Each suspected metal must be tested for and each entails a separate charge ($30-60 in this case). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images". You will find many charts with lots of data (some of them show multiple sources). By searching pages for this term you will find books having detailed sections on each of the metals. This can seem overwhelming, but typically you are only interested on one metal in a specific glaze (often cobalt or manganese).

Context: Are Your Glazes Food Safe or are They Leachable?, Attack on Glass: Corrosion Attack Mechanisms, Having Your Glaze Tested for Toxic Metal Release, USA DEP water testing info page, Chemical contaminants in drinking water, Leaching

Thursday 2nd April 2020

A terra cotta body fired from cone 06 (bottom) to 4

Terra cotta bodies are more volatile in the kiln than stonewares. They mature rapidly over a narrower range of temperatures, that process is accompanied by dramatic changes in fired color, density and fired strength. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP, cone 02 finds it at maximum density (and fired shrinkage). At cone 06 (1830F/1000C) it is porous and shrinks very little. But as it approaches and passes cone 03 (1950F/1070C) the color deepens and then moves toward brown at cone 02 (where it reaches maximum density). However past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). This is typical of most terra cotta clay materials.

Context: Maturity, Terra cotta

Wednesday 1st April 2020

This is what labs use to measure particle size

To measure particle size in a slurry or powder you need sieves. This is the most popular type used in labs. They are made from brass by a company named Tyler. The range of screen sizes for testing particle size is very wide (obvious here: the top screen has an opening of 56 mm, the bottom one 0.1 mm - the wires are almost too small to see). You can buy these on ebay for a lot less than new ones, search for "tyler sieve". The finer sieves (especially 200) are fragile and easily ripped. It is good to have a 50, 100 and 150.

Context: Sieve Analysis Dry, Sieve Analysis Wet, Sieve Analysis 35-325 Wet, Wet Sieve Residue, Particle Size Distribution of Ceramic Powders, How to Find and Test Your Own Native Clays, Particle Size Distribution, Ultimate Particles, Sieve

Wednesday 1st April 2020

Slaking. What is that?

A slaking clay bar (of a typical pottery clay). On the left the clay bar has been in the water for around 10 seconds. On the right, after a couple minutes, the rate of slaking has increased dramatically, the corners are disintegrating. Watching this process can be addictive! In about 20 minutes this bar will disappear into a pile on the bottom. Slaking happens most quickly when the sample is completely dry. High plasticity clays take longer (like this one). Slaking can be used to prepare clays for use: Dry the lumps, break them down using a hammer, put them in water, wait (less than 30 minutes for typical pottery plasticity), propeller mix, screen out impurities, then dewater to plastic state.

Context: Slaking

Wednesday 1st April 2020

Mother Nature's Porcelain - From a Cretaceous Dust Storm!

Plainsman Clays did 6 weeks of mining in June-July 2018 in Ravenscrag, Saskatchewan. We extracted marine sediment layers of the late Cretaceous period. The center portion of the B layer is so fine that it must have wind-transported (impossibly smooth, like a body that is pure terrasig)! The feldspar and silica are built-in, producing the glassiest surface I have ever seen (despite this, pieces are not warping in the firings at cone 6). I have not glazed the outside of this mug for demo purposes. I got away with it this time because the Ravenscrag clear glaze is very compatible. But with other glazes they cracked when I pour in hot coffee.

Context: B Clay, How to Find and Test Your Own Native Clays, Mother Nature's Porcleain, Clay and dinosaur country in southern Saskatchewan, Vitrification

Monday 30th March 2020

Fusion 360 drawing for glaze melt flow tester is available

We have promoted this device for many years as a way to compare glaze melt fluidity, surface tension, bubble retention, crystal growth, transparency, melting range, etc. Download the Fusion 360 file using the link below.

Context: A Low Cost Tester of Glaze Melt Fluidity, Melt flow test 5v1 Fusion 360 3D file, Melt Flow Tester 3D Scan STL file

Monday 23rd March 2020

Two stains. 4 colors. Will the guilty oxide please step forward.

We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company they need to know the chemistry of your glaze to help.

Context: ZnO, CaO, MgO, G2916F, G2934, Digitalfire Tech Tips PDF Handouts, Glaze Chemistry, Ceramic Stain, Opacifier

Thursday 5th March 2020

Feldspar applied as a glaze? Yes! The way I did it will change how you glaze.

Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and started adding the epsom salts (powdered). After only a pinch or two they both gelled. Then I added more water and more epsom salts until they thickened again and gelled even better. They both applied beautifully to these porcelains. The gelled consistency prevented them settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with epsom salts then any glaze will. You never need to tolerate settling or uneven coverage for single-layer dip-glazing again! Read the page "Thixotropy", it will change your life as a potter.

Context: Epsom Salts, Suspending pure feldspar and applying it as a glaze, Pure Custer Feldspar and Nepheline Syenite on cone 10R porcelain bodies, Thixotropy, Powdering, Cracking and Settling Glazes

Wednesday 4th March 2020

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