Our base glazes plus opacifiers on a dark burning body at cone 6
The body is Plainsman M390. These are commonly used base glazes. The top one is an MgO matte, next down is a calcium matte. They behave very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.
Tuesday 2nd June 2020
Laguna B-Mix on Steroids: Wedge in some Plainsman Fire-Red!
Both pieces have a transparent glaze, G1947U. The Fire-Red (a blend of Plainsman A1/M2 and St. Rose Red) was mixed as a slurry, dewatered to plastic form and then wedged in to the B-Mix (left piece has 10%, the other 20%, the bar in front shows the pure material). The A1 supplies most of the speckle, the St Rose and M2 impart the color. This addition does not affect the working properties of BMix (it still throws very well). An added benefit is that pieces dry better. Fired strength and maturity are minimally affected (porosity stays around 1%). With a 20% addition the surface of the unglazed clay is almost metallic. Silky matte glazes, like G2571A, are stunning on a body like this.
Tuesday 2nd June 2020
Reverse-engineer a commercial transparent glaze to get the recipe
The commercial cone 04 clear brushing glaze on the left works really well on our bodies so I sent it away to be analyzed (about $130). From that information and using my account at insight-live.com I was able to create a recipe, having the same chemistry, employing two Fusion frits (which amazingly supplied all of the fluxing oxides). In this cone 04 melt fluidity comparison they are almost identical (mine, G3879, has a little more surface tension). The Al2O3 and SiO2 levels would make, even a cone 6 glaze, jealous! So it should be very durable. And it has low thermal expansion (no crazing). With the bucket of dipping-slurry I made I can glaze a piece perfectly evenly in seconds rather than the normal 10 minutes of fiddling with a brush and three coats! I have used it on dozens of pieces, it's amazing. One caution: It is possible to duplicate a transparent glaze like this but not a coloured one (a lab could not analyze the complexities of the color, stain colors are about more than chemistry (firing method, particle physics). For colored glaze you have to do trial-and-error testing with stain additions to this base.
Context: G3879, A Low Cost Tester of Glaze Melt Fluidity, Getting a chemical analysis of your ceramic powder, How and where to have a glaze tested to learn its chemical analysis, This mug has waterlogged because it is not vitrified, Five low fire glazes: Which is the best?, Frit, Kovar Metal
Monday 1st June 2020
This pitcher is oozing a black goo after water sat in it overnight
Even after two weeks it is still sticky. This was purchased at an import store. What could this black goo be? It is likely a sealer that they use to make the porous clay water tight, perhaps an organic sugar. The clay is porous (and thus also weak) because they want to save energy by firing their kilns as low as possible. A water soluble sealer can be OK if the vessel is not used for storage. But it is not OK because there is another problem: The glaze is crazed. That is what is permitting the water to be absorbed into the body. That water is dissolving the sealer and bringing it out. There is yet another issue: The glaze could very well contain lead. Lead makes glazes melt low, so it is a great for saving energy. But not so great for producing safe ware.
Sunday 31st May 2020
Melt fluidity differences are not obvious by just comparing glazed ware
These two Plainsman M370 mugs were fired at cone 6, the left one with G2934 matte glaze, the right one with G2934Y4 matte. They look and feel identical in the hand. The two glazes have the same chemistry. But they employ different materials to source that chemistry. The secret of of the matteness is high MgO (magnesia content). In the glaze on the left that MgO is sourced by dolomite, a lot of it. The glaze on the right sources it from a special frit, Ferro 3249. The impact of this difference is visible in the melt flows, the fritted one is obviously melting and flowing better. On other clays, especially stonewares, the G2934 can have a dry surface that cutlery marks. Thicker applications make it worse. But the Y version exhibits no such issues. Its mattness, durability, cleanability and hardness are so good that it is being used in floor tile.
Friday 29th May 2020
Fusion 360, my choice for 3D modelling
Intimidation by the complexity of this type of software is the biggest obstacle you will face to learning 3D design (for 3D-printing). That being said, the new mission of AutoDesk, the leader in CAD software for 30 years, is to make it easy and universal! Fusion 360 has a lot of advantages. It is a standard. There is a simple learning curve via Tinkercad.com, videos on Youtube, easy online help and many freelancers to hire (at Upwork.com). It is free to qualifying users (teachers, students or people who earn less that $100k/yr), the fact that software of this kind of power and utility is actually available to anyone that wants to try it is amazing. Fusion 360 (and other 3D design products) cannot run 3D printers (3D slicers do that). Fusion 360 is very demanding on the processor and graphics hardware of your computer, typical laptops are not powerful enough.
Friday 29th May 2020
CMC gum solutions can go bad
That is why glazes containing CMC often need a biocide if they are going to be stored for extended periods. We made this one. The gallon jar of Laguna gum solution sitting next to this did not go bad, that means they have added some sort of anti-microbial agent.
Sunday 24th May 2020
Trafficked online recipes waiting for a victim to try them!
You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).
Saturday 23rd May 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 then analysed for trace levels of specific metals). Each suspected metal to be tested for 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" to find charts with lots of data. Searching pages for this term will find books having detailed sections on each of the metals. Typically you are only interested on one metal in a specific glaze (often cobalt or manganese). There are ways to sleep better (about the likelihood your glazes are leaching metals) if you cannot do this: Do a simple GLLE test. And avoid the online trafficking in hazardous recipes. Better to find a quality base glaze (matte and transparent) that works well on your clay body. Then add colorants, opacifiers and variegators; but doing so in a conservative manner.
Context: Are Your Glazes Food Safe or are They Leachable?, Attack on Glass: Corrosion Attack Mechanisms, Having Your Glaze Tested for Toxic Metal Release, Glaze Leaching Test, Concentrate on One Good Glaze, Want bright orange? Use a stain in your own base transparent recipe., Trafficked online recipes waiting for a victim to try them!, USA DEP water testing info page, Chemical contaminants in drinking water, Brandywine Scientific Lab Website, Liner Glaze, Leaching
Wednesday 20th May 2020
Why it is not a good idea to use straight stain
The logo on the left was rubber-stamped using and ink mix made of only glycerine and Mason 6666 black stain. The glaze is shedding off during firing. Multiple properties needed by a stamping ink are not present here. First, the stain dries as a powder, it has no hardening or bonding properties, glycerine is its only mechanism. Second, it is too concentrated, the black color is so powerful that it bleeds excessively into the overlying glaze. Third, it does not melt during firing so it does not bond with the body below. And, it either develops only a fragile interface with the glaze above, or sheds it off. The piece on the right mixes the stain 50:50 with a glossy transparent glaze (having 20% kaolin), it lays down better, accepts the overglaze layer better (because it has less glycerine), presents less problems in handling before glazing and it has no issues with the overglaze crawling off during firing. Black stains are potent, a 75:25 stain:glaze mix would work even better.
Friday 15th May 2020
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