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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

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

Drip glazing and bare outsides: Deceptively difficult.

thick glaze drips on ceramic vessels

Why? Glaze fit. Do these yourself and they might end up being glaze compression demonstration pieces. These are available on Aliexpress (as Drip Pottery, Drippy Pottery or Goopy Glazes) and they are made by a manufacturer that has close control of body maturity (and thus strength) and the capability to tune the thermal expansion fit of glaze-on-body. Glaze fit has to be better than normal because of the absence of an outside glaze. Too low an expansion and the compression (outward pressure) will fracture body (especially for thin-walled pieces). Too high and it will craze. And the glaze is thick, it will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They likely deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. And applied the glaze to preheated ware. If done right these pieces are a visual and technical achievement. However hobbyists, for example, often just brush multiple layers of commercial glaze that only by accident fits the body they are using. No wonder their pieces often end up as time bombs or crazed bacteria farms.

Context: Glaze at 1 7.., Why are these vessels.., Deflocculation, Glaze fit, Glaze Compression, Glaze thickness

Wednesday 12th June 2024

Glaze cracking during drying? Wash it off and then do this.

Glaze spider web cracking on drying

If your drying pottery glaze is doing what you see on the left, a spider web crack pattern, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. The first thing to check is water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing. But that was not the issue here. Glazes need clay to suspend and harden them, but if there is too much it can mean trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove it.

Context: Calcined Kaolin, Calcination, Crawling

Tuesday 11th June 2024

G2926B transparent glaze, proven reliable and durable

A hand made pottery mug with clear liner glaze

While colorful and layered glazes on the outsides of pieces get lots of praise and glory transparent or white glazes providing the functional surface on the insides of pieces often get little attention from potters. Really, what good is an attractive piece if the inside food surface is crazing, blistering, leaching or cutlery marking? Or if it onverts the piece into a time bomb? This cone 6 liner glaze, G2926B, is an example of how I found a recipe, recognized its potential and tuned and adjusted it to be better and fit our clay bodies and be more durable. 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 is well documented having a code number that Google indexes. Drinking from a mug having a quality and fitted functional surface instills pride in me as the maker. And it minimizes complaints from customers. The outside glaze? It is G2934 plus stain.

Context: G2926B, The Development of G2926B..

Thursday 6th June 2024

The glorious colors of the G2934Y cone 6 matte glaze

G2934Y with colors

These are slipcast from Plainsman M370C and fired to cone 6 using the PLC6DS firing schedule. The glaze is G2934Y, a recipe variant of G2934 (having the same chemistry but the MgO is sourced from a frit and talc instead of dolomite). It has a finer micro-surface texture and melts even better. Both recipes are adjustable, the degree of matteness can be tuned by recipe or firing change, or both. This is an MgO matte so it is very pleasant to the touch. It fires durable, has very good slurry and application properties as a dipping glaze. It has a very low thermal expansion (won’t craze on anything). It works really well with stains (except purples). It melts better than our G2926B glossy, even though it fires matte!

Context: G2934Y, G2934

Thursday 6th June 2024

L3954J black engobe on a cone 10 whiteware body

Black cone 10R engobe

The body is Plainsman H570 (0.5-1% porosity). Notice the EBCT test bars in front (engobe compatibility). These sandwich the body and the engobe together in a thin strip, differences in fired shrinkage curl the bar during firing (toward the higher shrinker). The straighter the bars fire the better the fit. My regular engobe for use on our buff stoneware, L3954N (Plainsman H550, 2-3% porosity), has lower fired shrinkage than this (since that body is less vitreous). This one increases the shrinkage (with 5% more nepheline syenite, 5% less silica and 3% less ball clay). This employs 10% Mason 6600 stain to produce the jet-black fired product.

Context: Mason 6600 Black Stain, L3954N, How to make a.., Absolutely jet-black cone 6..

Tuesday 4th June 2024

22 inch slabs successfully fired after a change. What?

22 inch tiles successfully being fired

Giant thin meter-square tiles are fired flat and crack-free by tile companies. How? Kilns that heat evenly from above and below (the tiles are on rollers). But these round tiles are being fired in an electric kiln, a device incapable of heating a large slab evenly. They are so large they reach almost to the outer walls. That means the outer edges receive direct radiant heat from the elements, this sets up a temperature gradient running from the edges toward the center. Passing such a piece up and down through quartz inversion thus creates a wave of sudden expansion and contraction moving through the piece. The artist was losing every one of these to dunting. It is not really advisable to even try this - but he was determined to do it anyway. One change in the process brought this one through: Slowing down to 50F/hr up and down through the quartz inversion (950-1150F).

Context: Electric Hobby Kilns What.., Drying and firing a.., Here is what it.., Ceramic Tile, Dunting and Cracking of..

Monday 3rd June 2024

A novel way to test glaze compression and glaze fit

Two coffee mugs, one cracked, the other shattered

These are made from L4005D red cone 6 stoneware. Both are cast and thin-walled (half of what a thrown piece would be). They were glazed only on the inside to encourage cracking/splitting if the glaze is under excessive compression (that is, the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a couple of taps with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press it together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z1. It spontaneously blew in half, with a loud crack, a few 5 hours after exit from the kiln. On further taps with a hammer these pieces shattered into dozens of smaller ones! The white glaze is certainly under too much compression. Obviously, neither is under any danger of crazing. Is the compression too great on the dark glaze? It did not shatter the way the white one did on further taps. And, another thicker-walled piece exiting the same kiln was glazed inside and out with that glaze. It was very strong. The lesson: Glaze compression, if not too much, is good for ware strength - but pieces must be glazed both outside and inside. And, thin ware like this must have better-fitted glazes.

Context: A M39 -compatible cone.., Glaze Compression

Monday 3rd June 2024

Saint Rose Red being delivered. Look what it does at cone 10R!

Dump trucks delivering the bright red lump clay to the Plainsman plant

We get this clay from St. Rose, Manitoba. Four tandem loads arrived this week. Just seeing the pile inspires me to make more pieces! It is a red fireclay and it is highly unusual. St. Rose Red has issues. They at first seem to be problems, but in combination they give it magic powers! It fires with very heavy iron speckling. The iron pigmentation is so high that it burns almost black at cone 10R. It has low plasticity. It shivers glazes: The vase on this picture lasted an hour after kiln exit, it spontaneously fractured because of the outward pressure from the under-compression glaze on the inside. But, by combining St. Rose Red with our more vitreous clays, which are highly plastic, we can make H440 and H443. A mix of only 45 St. Rose with 40 Ball clay and 15 feldspar produces a rustic metallic surface (like the cup shown). Such a body cannot be made from a low fire red clay (like RedArt), it would just warp and collapse in the kiln. It is the refractory character, heavy pigmentation, iron speckling and low plasticity of St. Rose that make metallic ware possible.

Context: Saint Rose Red, The stockpile of St.., Plainsman Red Fireclay Fire-Red.., Laguna B-Mix on Steroids.., Plainsman Fire-Red reduction fired.., Raw clay delivery in..

Tuesday 28th May 2024

A black engobe transforms the floating blue glaze over it

Floating blue over black engobe

This is M340 stoneware glaze fired to cone 6 using the C6DHSC schedule. The L3954B engobe fires deep black (it has 10% Mason 6600 black stain). The engobe was applied by pouring and dipping at leather hard stage (inside and partway down the outside). After bisque firing the piece was glazed inside using the base GA6-B Alberta Slip amber base. The outside glaze is Alberta Slip Rutile Blue GA6-C (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers).

Context: L3954B, GR6-M, G2826R, FLB

Sunday 26th May 2024

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