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).

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.

This is the G3948A recipe fired to cone 6 using our standard C6DHSC schedule. The color "breaks" to black where thinner around contours so it seemed like a natural that the inside glaze should be G3914A Alberta Slip black. The contour of the foot ring is important or the glaze will run onto the kiln shelf. My standard fluted ring foot is working well. Perhaps a better option would be to glaze the bottom inch or so with the black as a catch glaze.

Yes. Ancient Copper, as of Nov 2023, it is no longer available. Right is G3948A, our iron red (a publicly available recipe). Both of these have been fired using the C6DHSC slow cool firing schedule. As you can see the PC-56 crystallizes more, matting the surface in the process. But if cooled normally (e.g. using the PLC6DS schedule) it does fire similar to G3948A. Likewise, G3948A can be made to crystallize more if the iron oxide percentage is increased in the recipe (we use black iron, it is a little less concentrated than red but does not gel the slurry). The recipe offers excellent slurry properties when mixed as a dipping glaze. Our version uses Spodumene (which has 7% Li2O). Of course, lithium materials are very expensive these days, but that is what is needed for this effect. If you make a brushing glaze of it using our instructions only about 70g of spodumene is needed to make a 500ml jar. At current material costs, we could make 3 jars for $10 worth of powdered materials!

Left is G3933G1, it is part of a project to create an Alberta Slip and Ravenscrag Slip versions of our G3933A recipe (repeated issues with crawling was the motivation). During the process the silky matte texture was lost and thus the opportunity to add lithium. The glaze on the mug on the right, G3933G1, it the same except for the addition of 6% lithium carbonate. Lithium is a super powerful melter, turning this into a very reactive glaze! Our current lithium price is about 15 cents/g. To make a 500ml jar of this would require 330g of powder, at 6% 20g of lithium would be needed. That is $3.

Left: G3933E oatmeal based on Ravenscrag Slip.
Right: G3933 oatmeal based on a mix of G2934 matte and G2926B glossy base glazes.
The Ravenscrag version features several advantages. Most importantly much less tendency to crawl. It is also more responsive to cooling differences (more matte on slow cool, more glossy on fast cool). And, its recipe is adjustable, it is easy to raise the MgO if a more persistent matte is needed. And, it is easy to adjust slurry properties by changing the ratio of roast-to-raw Ravenscrag clay. And, it looks better, transmitting the brown body color on thin sections more effectively. And it gets better on refires.

Casey Larson, our shipper and a pottery enthusiast, found this while breaking lumps on a stockpile. This is a very unusual find. The vast majority of fossils we find are preserved in iron stone concretions in our A1 raw clay, the top layer. Layers below that are highly plastic, as their lumps weather (many of which arrive the size of microwave ovens) they shrink and break down into smaller and smaller sizes. But our 3D material (the majority ingredient in Ravenscrag Slip and the lowest layer we mine) is less plastic so the lumps shrink much less as they dry. This keeps larger ones intact and has preserved this beautiful fossil imprint. This lump has been bisque fired to make the impression durable, thus the lighter color.