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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

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

Monday 16th 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, Stain, Opacifier, Opacification

Thursday 5th March 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

Wednesday 4th 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

Are we collectively losing the simple ability to weigh out a glaze recipe?

Prepared glazes are starting to hurt our control, independence and sense of responsibility regarding the quality of our ware. Many potters are even losing simple abilities, concepts. Some potters are unaware of even what weighing out a glaze is. Some do not know how to recalculate a recipe total! Or even what a gram is. Or what ceramic materials are (like a frits, feldspars, koalins, or silica) and the purpose of each type. Or even where even to buy them? Or the difference between a suspension and a solution. Or what specific gravity and viscosity are? Is this a trend we want?

Context: What is the Glaze Dragon?

Thursday 27th February 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

Sunday 2nd February 2020

Why do these cone 04 and 6 clear glazes have so similar a chemistry?

The glaze on the left (as shown in my account at is a crystal clear at cone 04. The high frit content minimizes micro-bubbles. The high B2O3 melts it very well (this has 0.66 B2O3, that is three times as high as a typical cone 6 glaze). The recipe on the right is the product of a project to develop a low-thermal-expansion fluid-melt transparent for cone 6 (with added colorants fluid melts produce brilliant and even metallic results and they variegate well). While the balance of fluxes (the red numbers in the formula) is pretty different, look how similar the B2O3, Al2O3 and SiO2 levels are (yellow, red and blue backgrounded numbers in the formula), these mainly determine the melting range. That means that a fluid-melt cone 6 glaze is actually just a low temperature glaze being overfired to cone 6.

Context: G3879, G3806C, What Determines a Glaze's Firing Temperature?, Fluid Melt Glazes

Saturday 1st February 2020

Is that recipe you found online really what you think it is?

It contains significant Frit P-25, I googled that to Digitalfire, it contains 1.8% fluorine! When that gases off during firing are you really equipped to deal with fluorine gas in your studio? One answer is to substitute my own frits (which of course do not contain fluorine). I did the calculations in my account at (see the recipes side-by-side). In the formula, I substituted the F (fluorine) for a mix of CaO/MgO. In the recipe, I substituted Ferro Frit 3110 (it is also a high-sodium frit) for the P-25, then I juggled the rest of the recipe to match up the oxides. More Gerstley Borate supplies the lost B2O3, more ball clay supplies the lost Al2O3 and less silica is needed (because the ball clay brought along more). I ended up with a recipe that is going to suspend much better (more clay) and was able to eliminate the whiting (because the rest of the new recipe is supplying more CaO). This recipe has a high thermal expansion, that means crazing. Would it not be easier to simply add a red stain to a clear glaze you already have that is working well on your clay bodies?

Context: F, Trafficking in Glaze Recipes

Saturday 1st February 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 (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: Terra cotta, Soluble Salts, Drying Shrinkage, Vitrification, Bloating

Saturday 1st February 2020

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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

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

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