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Plainsman Clays is one of more than 90 ceramic suppliers in North America. But we are the only company that produces most of our products from clays that we mine ourselves. This affords an unmatched ability to maintain quality and supply (which we have done through a dealer network for 50 years).

Technical Tips Blog

Low fire mug survives two-foot drop on to cement

In slow motion. This is Plainsman Snow clay, a super white burning dolomite body. Fired at cone 04. Although low-temperature ware is not as strong as stoneware it has more elasticity. Other than a tiny chip out of the base and handle, this mug still has the ring of strength. Where does this piece get all this strength? 1. It is glazed inside and out. 2. Glazes adhere well to this porous body, forming a deep interface. 3. The compression under which the glazes exist not only resists crazing but adds a lot of strength to the piece. Notice how it bounces onto the handle also!

Context: Fired Strength

Friday 5th July 2024

Cone 10 Reduction with Alberta and Ravenscrag Slips

Outside is the GR6-A recipe (made using 50:50 roasted and raw Alberta Slip), there are no other additions. The inside is pure Ravenscrag Slip GR10-A (also a 50:50 mix of roast:raw), with no additions. The clay is Plainsman H550. The firing schedule is C10RPL.

Friday 5th July 2024

Variegating effect of sprayed-on layer of titanium dioxide

Pure titanium as a varieagant

The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However, on the outside the dried-on glaze was over-sprayed with a very thin layer of titanium and water (VeeGum can be used to help gel the sprayable titanium slurry and suspend it). The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile. Another advantage is that the variegation can be selectively applied in specific areas or as a design. This effect should work on most glossy glazes having adequate melt fluidity.

Context: Titanium Dioxide, GA6-D, Reactive Glazes, Crystallization, Glaze Layering, Variegation

Thursday 4th July 2024

A mug cracks before your eyes because of glaze compression

The glaze on this highly vitreous, thin-walled mug is normally perfect, it is under enough thermal compression to really increase ware strength. But, since this mug is glazed inside only, the compression is too great. While it looks OK, the glaze is constantly pressing outward, looking for relief. Watch as a tap with a spoon is enough to trigger a sudden crack. And it opens under the pressure, clearly revealing the piece was spring loaded. This is a simple test. A typical mug of this clay would survive hundreds impacts of this nature. Further, this did not happen just because it was not glazed on the outside. A mug with glaze under compression on the inside and under tension on the outside would fail this test even more dramatically.

Context: Glaze Compression

Monday 1st July 2024

The same glaze fires very differently depending on kiln cooling rate

Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the left, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the right. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the likely narrow temperature range they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.

Context: B2O3, Fe2O3, Ravenscrag floating blue color.., The matteness of this.., Alberta Slip GA6-A glaze.., Cooling rate drastically affects.., Same glaze fires hyper-matte.., The appearance of this.., Melt Fluidity, Glaze Chemistry, Crystallization

Monday 1st July 2024

Is inside-only glazing a good idea? We say it is not.

Glaze compression fractures porcelain mug

This mug is made from 325 mesh MNP, the strongest porcelain I have. Since the walls are of even thickness with no abrupt corners or contour changes and the glaze is thinly and evenly applied I thought I could follow a social-media-driven trend and glaze only on the inside. But I got glaze compression time-bombs waiting for hot coffee triggering! Three other mugs failed this same way! But four with this same glaze inside and out were fine. Why? The outside glaze counters the inside one pushing outward. And it closes crack initiation points.

I got lots of pushback on social media saying glaze compression problems are overblown. But I also got stories and pictures much worse than this (especially with thick and drippy glazes). But, some still feel that outside-only glazing can work by carefully tuning the thermal expansion fit between body and glaze. Or even by accident. Either way, there is still an elephant in the room: Glaze fit has to be just right - too much and pieces break, too little and the glaze crazes. That is a problem because it brings intolerance of even slight changes in body, glaze or firing.

Context: Drip glazing and bare.., Example of COE curves.., Glaze Compression

Thursday 27th June 2024

A down side of high feldspar glazes: Crazing!

A runny, amber, crazed glaze

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute the brilliant gloss and great color but the price is very high thermal expansion. Scores 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 can harbor bacteria and customers 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 possibly tolerate 10% of that. That would reduce running, improve fit and increase durability. Failing that, the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some glaze chemistry.

Context: Feldspar, Na2O, ChatGPT is completely wrong.., Cone 1 mug is.., Celadon Glaze, Calculated Thermal Expansion, Limit Recipe, Glaze Crazing

Wednesday 26th June 2024

Preparing clay test bars to measure plastic, dry and fired physical properties

Preparing bars for the SHAB, LOI and DFAC tests

This is being done for SHAB, LOI and DFAC tests. The clay was wedged thoroughly, rolled to 3/8 thickness (using the metal rods as gauges) and then cut to 4 1/4" by 1" bars. Code numbers and specimen numbers are stamped on each (these are needed to enter data into our account at Insight-live). For example, notice that the bars have specimen numbers from 1 to 6. These will be fired at six different temperatures (for a low-temperature body, for example, we fire cone 06, 04, 03, 02, 01, 1, 2). The data measured from each, including the temperature, will be entered for each bar (specimen). The 12cm dia. disk is being cut from 3/16" thickness. Notice how these clay tears as cut, this is an indication of the low plasticity of this native clay material. And notice the greenish color, that is typical of terra cotta clays.

Context: Shrinkage/Absorption Test, Code Numbering

Saturday 15th June 2024

3D-printed plaster mold natches, retainers and embeds

3D printed mold natches

Top left to right: The natch, the retainer, a fragment of a 0.8mm thick 3D printed mold shell and the shallow and deep receptacles that fit snuggly over it.
Lower left: The deep and shallow receptacles are embedded in a test section of plaster, the natches are ready to insert (head first or feet first). The natches have been glued in on the right.
Not shown: Cylindrical retainers that fit inside the embeds. These enable replication of an embed in a case mold to an embed in a working mold.

In some ways, these are preferable to the commercially available natches. First, the embeds enable flexibility in what will be inserted into either case or working molds (the natches, for example, are glued into the embeds). A key advantage of this, vs using commercial natches, is that working molds release from the case molds with flat matting surfaces - meaning they can be sanded to ultimate flatness for optimal fit (since a little warp can happen in the 3D printed block mold). Another advantage is that parametric drawings make it easy to change the sizes of all needed parts. This project is a testament to the accuracy of 3D printing - it is precise enough, on our Prusa MK4, that 1/10 mm is the difference between perfect fit and too tight or too loose.

Context: A demo for using.., Mold Natches

Thursday 13th June 2024

What is the simplest, most practical raku base crackle recipe?

A glazed tile showing the raku crackle effect

Many people suffer high-percentage Gerstley Borate "bucket-of-jelly" raku recipes they find online. Most of these are just transparent base recipes to which colorants are added. After years they found ways to tolerate this strange bedfellow. Now, a more normal material, Gillespie Borate, seems odd and is causing issues in the alternate reality "Ghastly Borate ecosystem". There is a better way. A frit is perfect for this application, Ferro Frit 3110 (or Fusion frit F-75). All it needs is 15% kaolin (e.g. EPK) to produce and easy-to-use recipe that is guaranteed to craze. The degree to which it crazes can be adjusted by trading off some of it for Ferro Frit 3249. We have assigned it a code number of L4264, a raku base transparent recipe. We have also catalogued some common recipes that people use and outlined the issues they have: L4264A, L4264B, L4264C, L4264D. Do you need a white? It is a simple matter of adding 10% Zircopax to this.

Context: Raku, Crackle glaze

Thursday 13th June 2024

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Plainsman Clays Ltd., 671 Industrial Ave, Medicine Hat, Alberta T1A 3L5
Phone: 403-527-8535    FAX: 403-527-7508