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Example of COE curves for body and glaze for sanitaryware

COE charts for sanitaryware glaze, body

These are from a sanitaryware plant. Long-term glaze fit is essential for their products. The glaze thus needs to be under some compression. That means the body must have a slightly higher coefficient of thermal expansion (COE) than the glaze. These two charts were created on the same dilatometer by the same person using well defined procedures (the glaze and clay each have their own procedures). A history of measurements and associated knowledge of how the data relates to the quality of the fired products provides a context to interpret these reports. In other words, technicians have learned that the difference shown here is what is required to achieve optimal glaze fit. Of course, some sort of database system (e.g. lab notebook, an account at is needed to record the history of testing to be able to compare the past with the present.

Context: Co-efficient of Thermal Expansion.., Sanitary ware

Wednesday 19th 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

This mug store is an encyclopedia on how to make each piece

WooCommerce running on the Wordpress platform is a common way to create an online store, it is more technical than some other options but it is also more flexibile. I want to be able to do something in my online store that no one else needs or wants to do. Let me explain.

Each mug description, as shown in the foreground page image, is complex having many links into that explain details of how it was made (e.g. glaze and body recipes, techniques, firing schedules, etc). The description on all 200 of them is formatted exactly the same. Each has a link to the permanent registration page, at, for that specific mug. Each has four Amazon-style product photos. Yet I did not enter any of these into the WooCommerce product dashboard - the complexity of doing that would have been impossible. There are two secrets to how I did this.

1. Notice the background image, that is a custom code database where I enter many details about every mug. A photographer, who has a separate login, takes the photos and uploads them to each mug. The search and bulk-edit tools in the database enable me to create consistency and accuracy.

2. Wordpress and WooCommerce both have an API. That means I can write code on the server hosting my database that talks to them through a back door. That code thus reads the data from my system and creates products on the WooCommerce site. It can also update products. It is remarkable to see it work!

Note: Don't buy any mugs yet, for now this is just a gallery. For some strange reason WooCommerce does not even have a setting for people like me that don't want to always sell!

Context: Tony Hansen pottery mugs..

Saturday 15th June 2024

1700F Frit Melt-Off: Who is the winner? Not the lead bisilicate!

Melted balls of 15 frits on a ceramic tile

These were 10g balls melted using our GBMF test. We fired at a temperature far lower than typical bisque, notice how many of them are already melting well! Frit 3602 is lead bisilicate. But it got "smoked" by the Fusion FZ-16 high-zinc, high-boron zero-alumina! Maybe you always thought lead was the best melter. That it produced the most transparent, crystal-clear glass. But that is not what we see here. That being said, notice the lead is not crazing but the FZ-16 is crazing badly, that is a problem for many applications using this frit, it relies on a high percentage of KNaO. Notice something else: Each frit has a distinctive melt fingerprint that makes it recognizable in tests like this. Want to get some of this frit for pottery? You can't, Fusion Ceramics doesn't want to handle retail sales of smaller quantities.

Context: Fusion Frit FZ-16, Ferro Frit 3602, Fusion Frit F-75, Ferro Frit 3124, Ferro Frit 3110, Fusion Frit F-524, Ferro Frit 3249, Fusion Frit F-69, Ferro Frit 3134, Fusion Frit FZ-164, Fusion Frit F-38, Frit LA-300, Frit B325, Ferro Frit 3195, Glaze Melt Fluidity -.., Ferro Frit 36 2.., Fusion Frit FZ-16 melt.., Comparison of frit melts..

Friday 14th June 2024

3D printed plaster mold natches and installation hardware

3D printing 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

A DFAC drying test disk of a terra cotta pottery clay from St. Ignacio, Sinaloa, Mexico

A clay drying performance test

This clay is used by traditional potters in the Mazatlan, Sinaloa, Mexico area. This DFAC test exhibits a very wide main crack and many edge cracks. These combine to indicate high shrinkage (a product of very high plasticity). Although the clay has some coarser grains that help channel water out, this is a very poor showing for drying performance - no large-scale manufacturer could tolerate this. But potters can and the Mexican craftsmen use it with success! How? By adapting their drying procedures. The high dry strength of this material is also a factor that helps prevent cracking. Almost any clay can be dried if the process is done evenly. And almost any can be cracked if done unevenly enough. This DFAC test is also a good indicator of the amount of soluble salts present (the slightly darker band around the perimeter), this is minimal so not scumming should appear on fired products.

Context: Drying Factor, Sulfates

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

Laminations: Will a studio pugmill solve the problem?

This company was plagued with drying cracks in their solid porcelain pieces. After some time they discovered that the deaired plastic material received from their suppliers had laminations (revealed in a cross section cut of the slug). Since they were not wedging, but simply inserting the clay into their hand extruders and presses, these laminations produced built-in weaknesses - the stresses of drying later exploited these. The obvious fix seemed to be to buy a mixer/pugmill to remix the clay. But that did not work. Why? Commercial pugmills commonly have multiple shafts, hundreds of blades, large powerful motors, separate mixing and vacuum chambers, shredders, high-compression heads, etc. Many small studio pugmills have none of these features. They are still great for recycling and mixing clay that will later be wedged. But for the machine-forming purposes of this company, this pugmill actually made the laminations worse! Would a deairing pugmill have solved the problem? There is still room for caution will studio equipment in production settings.

Context: Laminations, Pugmill

Tuesday 11th 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

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