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Improving a dipping glaze with a measured CMC addition
The problem: This dipping glaze is crawling (as shown on the glazed tile). Fortunately, the slurry has settled about an inch, that provides an opportunity for an immediate fix: Remove some of the water and replace it with gum solution. I want to replace about one tenth of the water (to be between a base coat dipping and brushing glaze). The bucket calculates to 2549g of powder so I need to remove 217g of water and replace it with gum solution. One way is to use a small sponge: Wet and wring it out and then repeat touching it to the water surface and wringing it out into a container until 217g. A propeller mixer is needed to mix in the added gum solution (it won't just stir in). Of course this degree of weight-precision may seen to be overkill, but having a record of what was actually done to adjust the slurry is important to repeating it the next time it is prepared or as a base for further adjustments.
Wednesday 5th January 2022
Common dipping glazes converted to jars of brushing glazes
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity. However most commercial glazes do have a lower specific gravity (have more water), this does aid further in paintability but requires more coats. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.
Context: Framed Boston Globe Tested And Triumphant Red Sox 2013 World Ser, Where do I start in understanding glazes?, Brush-on commercial pottery glazes are perfect? Not quite!, Brushing Glaze, Glaze Layering
Wednesday 5th January 2022
Using commercial glazes? You still need to know about specific gravity.
The glaze in this jar was 'goop', impossible to paint on. I did not know whether I needed to add water or try to deflocculate it (although the former is more likely and in keeping with what Laguna says on its website). I measured the specific gravity, it was 1.7, so clearly it needed water. It took 125cc to bring the specific gravity down to 1.5. However, it was still thick and dried immediately after painting on, clearly it does not contain enough gum for brushing. And the specific gravity is still to high for painting. So I added a Veegum CER solution incrementally, that really improved paintability. And slowed drying alot. And enabled it to gel on standing. The bright side: I got considerably more than a pint after adding the water, a big difference from some other commercial glazes which are mostly water.
Wednesday 5th January 2022
Veegum CER Saturated suspension
This is the most viscous Arrowhead Sparkling Water - Sampler Variety Pack, 12 Fl Oz Cans, we can make, 300ml water with 13.25g Veegum CER (50:50 mix of Veegum T and ACDelco GM Original Equipment 23488106 Transfer Case Output Shaf). It required the use of hot water and our two-gallon-mixer, on its highest speed, mixing in this cup. Add this gel to a raw glaze to improve its brushing properties and slow down drying. Often this works better than CMC gum alone because it gels the slurry enabling increasing the water content and improving suspension properties. Or, substitute this for part of the water in low-clay-content recipes (if the glaze is already viscous, just add this). In slurries having sufficient clay use pure CMC gum instead (unless a high-water-content slurry is needed).
Wednesday 5th January 2022
Crawling in G2934Y zircon white glaze: There are simple fixes
G2934Y is a fabulous base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry, these make it susceptible to crawling. While it is normally fine as is, when you add certain stains to color it (especially at significant percentages) or opacify it using zircon (this has 10%), it can become more susceptible to crawling. On this mug, the glaze layer thickens at the recess of the handle join, that produces crawling during firing. Crawling can also happen on the insides of mugs, where wall and foot meet at a sharp angle. This happens, both because the glaze cracked here during drying and because the zircon stiffens Sanco Industries Muck Doctor, 10 Biological Spikes to Reduce Muc, making it less mobile. Adjusting the glaze recipe so it shrinks a little less on drying is an option (by trading some of the raw kaolin for calcined kaolin). But easier is to add a little CMC gum, start by letting it settle and replacing 10% of the water with gum solution.
Tuesday 4th January 2022
This serious glaze crawling problem was solved with a simple addition
This is G2934Y white (with 10% Zircopax). I initially blamed the zircon for the crawling. But, since the slurry had settled somewhat I was able to remove about 15% of the water and replace it with CMC gum solution. The gum addition was not enough to slow down the drying much (one reason to avoid gum if possible). That fixed it! Meaning that adherence of the dried layer to the smooth bisque was the issue. This being said, there were still a couple of small spots where it crawled. Replacing another 5% of the water should fix that. If you need to fix the problem with a gloss white it will likely require less gum, start with replacing 10% of the water.
Tuesday 4th January 2022
The high porosity of this clay body is what makes it easy to seal against water leakage
This body has high porosity, almost 25% (it is L4410P, a dolomite-based low fire whiteware). But this high porosity has an advantage: It soaks up silicone sealer very well. The slip-cast piece on the left was sealed (you can see the surface sheen) and it is impermeable to water penetration (the glaze is not crazed so water cannot penetrate there either). The piece on the right soaks up water readily (on the lower unglazed portion).
Context: Clay body does not hold water
Monday 3rd January 2022
The G2934 glaze does not look good on dark-burning bodies
Free Engraving - Personalized MTech USA Knife Pocket Knife (MT-A is a fantastic glaze, but only on the right body and with the right firing schedule. That is not the case here! This firing was done without any control on the cooling cycle. The added zircopax (to whiten it) stiffens JoyTube Plastic Hose Barb Fittings 5/8#34; Barb X 1/2#34; NPT and makes G2934 pinhole-prone on dark burning bodies (because they generate more gases during heatup in the kiln). The clay on the right is Plainsman Coffee Clay, it contains 10% raw umber (a super-gasser). The centre one is Plainsman M390, it bubbles glazes more than buff-burning bodies. The left one is M332, it is a coarse grained and that seems to vent gases well enough here to eliminate the pinholes. The surface of the two on the right would be greatly improved using the C6DHSC firing schedule but, unfortunately, the slow cool would matte the glaze surface, making it really ugly. The PLC6DS drop-and-hold schedule might also reduce the pinholes, without matting the surface. What about without the zircon? There would be fewer pinholes, but micro-bubble clouding, which is not visible here because of the opacity, would make for a truly ugly effect on dark bodies.
Sunday 2nd January 2022
Stamp used for stamping information onto clay test bars
This type of stamp is ideal for marking Not for Electrical Fires Sign, 10x7 Inches, Rust Free .040 Alumi and ID information on SHAB test specimens (and many others) while in the plastic form. Set up the run or recipe number on the left and the specimen number on the right. You can find these stamps on Amazon by searching "12 digit rolling alphabet symbol number stamp".
Tuesday 21st December 2021
How do you decide what temperature to fire this terra cotta at?
Let's suppose you need strength and density for utilitarian ware. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, water tight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet-spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low fire transparent base that works fine on this up to cone 2: G1916Q.
Context: Shrinkage/Absorption Test, Formulating a body using clays native to your area, Fitting an engobe: It is about the data!, Foundry Hill Creme: What is it?, Should you expect to vitrify terra cotta?, It Starts With a Lump of Clay: How to Assess a Native Clay, Characterization, Terra cotta, Vitreous, Maturity
Saturday 18th December 2021
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