The terra cotta casting body on the right, L4170B, normally casts really well (even better than the M370 on the left). Even though we have made this many times … today it is not working right. It took twice the amount of time in the mold to build up the needed thickness. It took three times the normal amount of time to release from the mold, when it finally did it wanted to turn inside out on pour (note the indent in the side). It also came out of the mold very soft and pliable. After drying, the surface, especially around the rim, has a hard film, it is difficult even to scratch. While the slurry itself is fluid and does not settle, it has the consistency of syrup. The problem is clearly over-deflocculation - this slurry is normally easy-to-deflocculate and performs very well. How did this happen? We are finding our new shipment of Darvan is more potent (therefore not as much is needed). Darvan has a shelf life, 2 years, the jar we were using was likely older than that thus more was needed.

This is another example of the flexibility potters have compared to manufacturers. These 3D-printed gizmos are stuck onto this beer bottle mold using the casting slip. Dipping their flat surfaces and attaching them takes seconds. Another feature of this mold for potters only: There are no notches (the halves were poured into disposable 3D printed PLA masters - and mate perfectly). Using the rubber band to hold them together was not ideal because realignment of the halves damages the square inside edges. By using this method the mold halves can be aligned accurately. The 3D printed pouring spout is likewise attached using the slip (it also helps hold the mold halves together).

There is nothing like feeling the texture of a clay body on a potter's wheel to judge the suitability of its particle size distribution. What you feel is sometimes different than PSD data may suggest. That texture can be recorded in your Insight-live account as a photo if you snap a closeup of the rim like this. I am testing the plastic character of a new mining of one of our clay materials and need to judge its texture in comparison with the existing one. In this case, these side-by-side photos confirm my other observations.

This glaze is 49% Wood Ash, 24% Soda Feldspar and 27% Ball Clay. 10 copper carbonate and 10 manganese dioxide are added to that. This beautiful sculpture was made by Dan Ingersoll, aesthetically this glaze is perfect for it. But there are two red flags here. Significant manganese and copper metal fumes are certain to be generated at cone 10 (they are seriously not healthy) so anyone using this must be very careful. But there is something much more serious - this glaze is being used on functional ware. Copper is well known to destabilize other metals in the fired glass. This 10:10 combination is a perfect storm for leaching heavy metal into food and drink. This is not an argument for the use of commercial glazes, it is one for common sense application of the concept of limit recipes.

Would you like to be able to use your own found-clays, ones native to your area or even your property, in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do exactly this. I will use ordinary tools that any potter either already has or can buy at low cost. We will describe this clay in terms of plastic clay bodies and common ceramic materials that most potters already use. The potter who submitted it has worked enough with the material to suspect it has potential and he wants to know how to best utilize it (e.g. at what temperature, with what glazes, mixed with what, processed in what way). In technical terms what we are doing is called "characterization".

This is G3948A (similar to the popular Ancient Copper product). To get this stunning result it needs to be applied thickly. Therefore it runs a lot. But the catcher glaze on the bottom cm of these mugs has stopped the flow. The catcher is a glossy black glaze and is hardly noticeable. I use G3914A as the catcher but Amaco Obsidian would also likely work. The inside glaze, G2926B, is one I have tested and developed to fit our clay bodies really well.

G3948A is a cone 6 iron red. This sample is firing using the C6DHSC schedule. It is a reactive glaze in more ways than one. This closeup reveals just how much is happening on that fired surface. The recipe contains spodumene, an expensive material, but clearly it is worth it.

The glaze defects are caused by precipitates that have formed in this glaze slurry within days of batching it. They are refractory and do not dissolve in the glaze melt - creating a defect that is unrepairable. In industry, glazes are batched and sieved as an ongoing process but in pottery and hobby ceramics they are stored for months or even years after batching. It is normal to have to sieve these slurries every few months but in recent years the precipitates form more quickly. Frits are theoretically insoluble, but in practice, they are not. Frit quality is determined not just by careful control of the chemistry but also of the smelting, mixing and water-quenching processes.

This thrown piece has thin walls and a thick base. A thickly applied inside glaze. No glaze on the outside (showing off the beautiful red body color). These factors are a recipe for glaze compression failure. And that is what has happened! But this has worked for the potter in the past! So what is needed to continue doing this unrecommended technique and get away with it? Thicker walls. Thinner base. Thinner glaze application on the inside.

Right: My attempts to apply a layer of watery white underglaze to leather hard clay.
Left: After adding about a gram of Epsom Salts (and stirring thoroughly to dissolve) the slurry became thixotropic and gelled enough to enable applying a thicker layer with no problem. While this works on leather-hard ware, a high specific gravity slurry like this would not be ideal for application on bone-dry ware (unless significant time is allotted for drying between coats).