Piedmont Pottery

This would probably work, but the crazing you will get with most commercial crackle glazes will be much finer than the example photos you posted. You will need to go over the pot with India ink after firing to get dark color into the cracks.

Wollastonite has a much lower LOI than whiting (1.6% vs 43.9%) and has a lower thermal expansion coefficient (9.0 vs 14.8) (data from DigitalFire). Wollastonite also provides silica in addition to calcium. My glazes are high silica/low expansion by design, and wollastonite gets me there. And finally, Wollastonite cost me $0.59/pound the last time I bought it, whereas whiting cost me $1.17/pound. I'm sure I could rework my recipes to substitute whiting, but there is a certain amount of inertia involved in doing that, as I'm generally happy with the way my glazes are behaving on my clay.

Some time ago I posted about challenges with wollastonite clumping. I have now found a process which is helping with this issue. I utilize one of these flour sifters when weighing out the wollastonite. I sift the material directly into a pan on the balance until I have the necessary amount. It's pretty low tech, but it works. Perhaps not feasible for large-scale batches, but I'm usually making 1-2kg batches of most of my glazes, and it works fine for that.

It’s a white stoneware clay, so not super high in iron, but I suspect there’s some. The town doesn’t report iron levels in their annual drinking water quality report, but I’ve not seen any rust staining in the toilet or sinks at the studio.

The material survived the bisque firing, strongly suggesting it was not organic. It sure looks like iron oxide to me.

I'll fire some chips in the bisque kiln this weekend. If it is tannins they should burn out. This is also city water, not well water, so I expect only low levels of tannins.

That's a possibility, but I would expect polyphenols in water to have a net negative charge, and therefore unlikely to bind to polypropylene.

That's a good point. I haven't really tested to see if it's iron or something else, but it sure looks like iron. I may try firing one of the chips of dried clay that has the material on it to see if it behaves like iron oxide.

I haven't noticed this happening on other bucket types, but I have a lot of this kind of bucket, so I haven't really used many others for reclaim.

For me it would be faster to just throw the plates on the wheel from the start rather than using the slab roller. However, I do find that a foot ring adds stability to plates during firing. Have you considered using a slump mold instead of throwing the slabs?

If the controls are corroded you may want to look at an external computer controller. That is what I did on my old Duncan 1029. It will cost more up front but will really help with more consistent firings. You will have to drill a small hole for a thermocouple to go through. all the infinite switches are set to Hi, or bypassed altogether. I did leave the kiln sitter inline to have a failsafe option.

I haven't had that problem with mine, but I've found Peter Pugger technical support to be very friendly and helpful when I've called about other issues. They should be able to offer suggestions.

Will your program accept multiple log files for long firing time programs? I often have two, sometimes three log files per firing, depending on the program. Also, I'm hoping that your program will be compatible with first generation Genesis log files. I'm really looking forward to your finished product.

I installed 2 mini splits in my kiln room and they work well.

Are you bisque firing after the underglaze and before the transparent, or are you single firing? If you are bisque firing first, how well adhered does the underglaze appear to be before applying the transparent glaze?

This is interesting, in that the calculated COE is low enough that I wouldn't expect it to craze on any of the clays I use. Regardless, my usual fix for glazes with too high an expansion coefficient is to add a bit of talc. I'd begin with maybe 250 g talc added to the amounts you listed as a starting point and go up from there.

But it only takes a little bit to fix a batch of short clay, so not too bad overall.

Veegum T is the magic additive I use when a batch is too short. I keep a container of hydrated Veegum T next to the pugmill so I can just add a small scoop to the pugmill if I need too. I am also very diligent in recovering as much of the fines as possible when reclaiming.

I substitute strontium carbonate for barium carbonate, 0.75 grams strontium carbonate per gram of barium carbonate in the original recipe. You will probably have to adjust this a bit, and you will see some shifts in color of the glaze. Strontium is non-toxic, as opposed to barium.

This definitely happens with lead glazes. Lead is very volatile at glaze temperatures and will bind to the kiln brick, and then a portion of it will re-volatilize on subsequent firings and will bind to everything in the kiln. For other potentially toxic metals the extent to which this can happen is dependent on the volatility of the metal at firing temperature.

I routinely use plastic cat litter buckets for collecting and transporting clay slop and trimmings. I have observed that in areas where the slip dries out along the walls the plastic is coated with red iron oxide, which also adheres to the dried clay. This is with a white stoneware clay, not a brown or red clay. So it appears that the trace amounts of iron in the clay are binding to polypropylene surface of the bucket. This makes sense because the surface charge of polypropylene is negative. I typically throw the dried clay flakes that are covered with iron oxide in with brown clay waiting to be reprocessed. Over time, my reprocessed white clay will have a lower concentration of iron, although I don’t think it will make a noticeable difference in the color of the clay, as I don’t think it’s removing more than a few hundred milligrams of iron per batch.

Delayed cracks can be indicative of a glaze with a thermal COE lower than the clay body. You might try to increase your glaze COE a bit to see if the cracks still happen.

It looks to me like you had enough organic materials in the kiln wash to create a reducing environment in the kiln and you got a lot of carbon going into the surface of everything. If that is correct, refiring with plenty of air available, e.g., peeps open or lid propped open just a bit, should burn off the trapped carbon. Fire it up to your normal working temperature.

Instead of using a lubricant we put the clay to be extruded into a thin plastic bag open at the die end. Clean up is just the die and the last few centimeters of the extruder tube.

E-6000 adhesive is what we use when customers come into the gallery for repairs like that. Needs to cure in place for 24 hours. Excess can be removed with a razor blade.