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Monthly Tech-Tip from Tony Hansen

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How could only a 5% fine grog body be suitable for such large pieces?

Louise Solecki Weir working on one of her large sculptures. Sculptors can be passionate about the clay they use. For good reason, they have a lot to lose. While it might seem that Louise would be most concerned about drying shrinkage and drying performance (resistance to drying cracks), not so. To her, the ability to re-wet sections that dry out is paramount. And she has learned to overcome drying challenges posed by the high plasticity to benefit from the smooth texture, workability and rewetability it offers. How plastic is this clay (Plainsman F96)? It is a five-equal-parts-mix of silica sand/grog, ball clay, Lincoln fireclay, a low fire red clay and a medium fire red clay (there is no feldspar or silica). All four of the clays are highly plastic to super plastic. The body's drying shrinkage would be 8% if it was not for the 20% aggregate (a mix of fine 75 mesh sand with a small complement of fine 40 mesh grog) that reduce it to 6.5%. These offer a far higher surface area than coarse grog and provide channels for water to re-enter. If you would like the recipe of this body (non-production) please contact us.

Context: F-75 Silica Sand, Plasticity, Grog

Saturday 6th March 2021

The next time I buy a bunch of materials to test an undocumented online recipe, slap me!

A matte glaze melt flow that is not moving at all

Look at recipes before wasting time and money on them. Are they serious? This is a cone 6 GLFL test to compare melt-flow between a matte recipe, found online at a respected website, and a well-fluxed glossy glaze we use often. Yes, it is matte. But why? Because it is not melted! Matte glazes used on functional surfaces need to melt well, they should flow like a glossy glaze. How does that happen? This recipe has 40% nepheline syenite. Plus lots of dolomite and calcium carbonate. These are powerful fluxes, but at cone 10, not cone 6! To melt a cone 6 glaze boron, zinc or lithia are needed. Boron is by far the most common and best general purpose melter for potters (it comes in frits and gerstley borate, colemanite). Other melters used less often are zinc and lithium (or the fritted forms). The lesson: Look at recipes before trying them, we use the term limit recipe to describe the skill of being able to eye-ball a recipe and quickly assess if it is ridiculous or not.

Context: B2O3, A Textbook Cone 6 Matte Glaze With Problems, Limit Recipe, Matte Glaze, Glaze Recipes

Wednesday 3rd March 2021

Texas talc (left) and Montana talc (right)

Texas talc contains some amorphous carbon. The carbon is not stand-alone, but as CO2 in the dolomitic part of the ore. It produces 7% LOI between 750-850C. Even though the color is so much darker in the raw form, it fires whiter!

Context: Amtalc-C98, Pioneer 2661 Talc, Silverline 303 Talc, Talc, Natural Talc C-98

Wednesday 3rd March 2021

Using Glaze Chemistry to replace Ferro Frit 3134 in three glaze types screenshot of three glazes with ferro frit 3134

Don't listen to people that say you can just replace frit 3134 with 3124 in glaze recipes. That is wrong. Frit 3124 has five times the amount of Al2O3 (the second most important oxide in glazes) and half the amount of boron (the main melter). The glaze chemistry approach is much better, and easier than you think. To be able to do it you need two other Ferro frits, 3110 and 3195. As it turns out, Frit 3195 is more important than is 3124! A key goal in the way it is done is to end up with at least 15% kaolin (to suspend the slurry). I have chosen three types of recipes to demonstrate, dealing with each requires a unique approach. Two of the calculations produce improved slurry properties and one a recipe of significantly lower cost. Stay tuned for a video on how to do this in your account. If you have a recipe that needs this, get an account, enter it there and I can help you do the calculation.

Context: Ferro Frit 3134, Ferro Frit 3195, Ferro Frit 3124, Is Ferro Frit 3124 a viable substitute for Frit 3134, Do you know the purpose of these common Ferro frits?, Substitute Ferro Frit 3134 For Another Frit

Thursday 25th February 2021

Wanna throw porcelain plates with thick bottoms and thin rims?

Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily from the plaster, even though the bat had been dry. When I did get it off the base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.

Context: Drying Ceramics Without Cracks, Why throw on a plaster bat when making larger pieces?, Drying Performance, Clay Cracking During Drying

Wednesday 24th February 2021

Fired bars of a micro-fine bentonite used in porcelains!

Fired bentonite test bars are dark brown

Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!

Context: HPM-20 Volclay Bentonite, Bentonite, Grolleg Kaolin, Efflorescence

Monday 22nd February 2021

G2926B transparent glaze, proven reliable and durable

A hand made pottery mug that rivals the quality of industrial ware

While colorful glazes on the outsides of pieces get lots of praise and glory, the transparent or white providing the functional surface on the insides of pieces often gets little attention. Really, what good is an attractive piece if the food surface is crazed, pinholed, blistered, leached, cutlery marked. This liner glaze, G2926B, is special at cone 6. It is a good example of how I found a recipe, recognized its potential and tuned and adjusted it to be better. It has proven itself as a base to host all manner of colorants, opacifiers and variegators. One of the reasons it is so widely used is that it has a story, it is well documented, with a code number that Google indexes. Drinking from a mug having a quality functional surface instills pride as its maker. And it minimizes complaints from customers.

Context: The Development of G2926B Cone 6 Clear Glaze

Monday 22nd February 2021

Milk as a glaze! How is that possible?

Clay tiles, one unglazed, one carbon glazed

After watching a youtube video (link below) about a Karelian potter, who uses this technique to make cookware, I could not wait to try it. He unloads the ware from his kiln (which appears to be a standard electric top loader used by potters in the west), and while still hot he immerses pieces in a bucket of milk for a few seconds. When he withdraws them they are steaming. I mixed some 2% milk and cream (to get closer to the whole milk he was using) and cold-dipped an 1850F bisque-fired jar and tile (of Plainsman L210) for about a minute (to enable it to soak in as much as possible). The potter claims to fire his ware to 300-350 degrees. I fired 500F/hr to 612F (350C), then held for 10 minutes and shut off to free fall. And it worked beautifully, high enough to get lots of carbon (which is only on the surface), not high enough to burn it away. The surface is smooth and pleasant-to-touch, it is odor-free. The potter claims it retains this surface over many years despite repeated oven use. This clay body, L210, is well suited since it is very fine-grained and fires to such a smooth unglazed surface. And the carbon makes it much better. Indigenous cultures throughout history have learned how to prepare, cook and store food in terra cotta clays like this, they withstand thermal shock better than vitrified stonewares and porcelains. Of course, more testing is needed, I will report as I proceed.

Context: Karelian potter produces glossy black pottery using milk as a glaze, Casein on Wikipedia, PolyWhey coatings by Vermont Natural Coatings, Terra cotta

Saturday 20th February 2021

A down side of high feldspar glazes: Crazing!

A glaze cone displaying a runny, amber, crazed

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could likely tolerate a 20% addition of that quite easily. That would reduce running, improve fit and increase durability. If the crazing does not stop the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some chemistry in your account.

Context: Na2O, Crazing, Celadon Glaze, Calculated Thermal Expansion, Glaze Crazing

Saturday 20th February 2021

A glaze is showing unwanted streaking. Why?

A mug with streaks of lighter blue on a darker blue background

This is a fluid melt cone 6 glaze with colorant added and partially opacified. It runs into contours during firing, thickening there (notice the darkening around the logo), this is a desired visual effect. However, notice that drips and runs coming down from the rim, they are producing darker streaks. This is an application issue. Glazes that fasten-in-place too slowly will drain unevenly on extraction from the bucket (after dipping). This can be solved by making a thixotropic slurry. If bisque ware is too dense, glazes have a more difficult time fixing-in-place in an even layer, especially if they have no thixotropy. If glazes lack clay (e.g. less than 15% kaolin) they do not gel as easily. Slurries containing too much gum dry slowly and drips are almost unavoidable. If the problem is too much melt fluidity, choose a more stable base glaze can really help. Just because melt fluidity is less does not mean that it will be less glossy.

Context: Fluid Melt Glazes, Opacity

Friday 19th February 2021

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