PeterH

Back up to his posting on this thread, and click on the big P. Then click on the Message icon, and you are messaging him -- by email so he doesn't even have to access the forum to see it. ... keep an eye on the icon at the top of the forum page to catch any personal messages addressed to you (it changes colour).

Are you sure that this isn't an indirect reference to heat-work? Perhaps in the context of unachievable ramp speeds.

These seem to be "wild" North Carolina clays (singly or as mixtures?). I've no idea if/where they might be bought, or how they might be approximated from commercial clays. Jeff makes a good point: if you are approximating the look of these clays then stains could simplify the process, especially if you wanted a palette of colours. From Wild Clay: The Story of NC Clay https://www.woodfirenc.com/wild-clay Sorry about the text size. Also see https://bandanapottery.com/materials Our main clay body… Our main clay body is mostly (75%) comprised of a mixture of two clays from our area: A coarse, red clay that is near our home in the higher mountains, and a more plastic grey clay that settled near a creek bed further down the mountain. We mix these wild clays into a slip and pour them through a screen into a large feed tank. The screening process is one of the first aesthetic and expressive decisions we make- the size of the screen determines the final landscape of particles in the clay beneath our slips and glazes. While dark in color, both these clays are so refractory in their pure state that we still need to add feldspar to the body to make it vitrify and hold water at stoneware temperatures. We also found that we need to add small amounts of ball clay, sand, and silica to our clay body to be durable and make the glazes fit. This brings up an important and interesting question- how much do we tame a clay to meet our functional expectations, and how much do we adapt and learn from the beautiful (and limiting) qualities that drew us to the wild clay in the first place? PS Re Joseph's mention of Grogeewemee 10, here is a picture

These seem to be "wild" North Carolina clays (singly or as mixtures?). I've no idea if/where they might be bought, or how they might be approximated from commercial clays. Jeff makes a good point: if you are approximating the look of these clays then stains could simplify the process, especially if you wanted a palette of colours. From Wild Clay: The Story of NC Clay https://www.woodfirenc.com/wild-clay Sorry about the text size. Also see https://bandanapottery.com/materials Our main clay body… Our main clay body is mostly (75%) comprised of a mixture of two clays from our area: A coarse, red clay that is near our home in the higher mountains, and a more plastic grey clay that settled near a creek bed further down the mountain. We mix these wild clays into a slip and pour them through a screen into a large feed tank. The screening process is one of the first aesthetic and expressive decisions we make- the size of the screen determines the final landscape of particles in the clay beneath our slips and glazes. While dark in color, both these clays are so refractory in their pure state that we still need to add feldspar to the body to make it vitrify and hold water at stoneware temperatures. We also found that we need to add small amounts of ball clay, sand, and silica to our clay body to be durable and make the glazes fit. This brings up an important and interesting question- how much do we tame a clay to meet our functional expectations, and how much do we adapt and learn from the beautiful (and limiting) qualities that drew us to the wild clay in the first place?

Personally I doubt it's practical, but this at least tells you not to overheat it (see the last sentence in the quote). Might be worth a try in a domestic oven. https://en.wikipedia.org/wiki/Plaster Chemistry See also: Calcium sulfate § Hydration and dehydration reactions Gypsum plaster, gypsum powder, or plaster of Paris, is produced by heating gypsum to about 120–180 °C (248–356 °F) in a kiln:[18][13] CaSO 4 ⋅ 2 H 2 O ⟶ heat CaSO 4 ⋅ 1 2 H 2 O + 1 1 2 H 2 O ↑ {\displaystyle {\ce {CaSO4.2H2O {\overset {heat}{{}->{}}}{CaSO4.1/2H2O}+ 1\!1/2 H2O ^}}} (released as steam). Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water. CaSO4⋅12H2O+112H2O⟶CaSO4⋅2H2O{\displaystyle {\ce {CaSO4.1/2H2O + 1 1/2H2O -> CaSO4.2H2O}}} Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time.[19] When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume. It is used in making casts for statues, toys, and more.[19] The initial matrix consists mostly of orthorhombic crystals: the kinetic product. Over the next 72 hours, the rhombic crystals give way to an interlocking mass of monoclinic crystal needles, and the plaster increases in hardness and strength.[20] If plaster or gypsum is heated to between 130 °C (266 °F) and 180 °C (350°F), hemihydrate is formed, which will also re-form as gypsum if mixed with water.[21][22] On heating to 180 °C (350°F), the nearly water-free form, called γ-anhydrite (CaSO4·nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C (480°F), the completely anhydrous form called β-anhydrite or dead burned plaster is formed.[19][22]

We seem to be talking at cross purposes. I was commenting on the use of sodium carbonate/bicarbonate for adjusting the rheology of a clay slip. While you seem to be adding sodium carbonate/bicarbonate to the surface of a pot then heating it: decomposing them to sodium oxide which eventually forms part of a "salt glaze". Min (with a like from Callie) commented on another thread How was your soda ash stored? If not kept sealed up in an air tight container it will change from sodium carbonate into sodium bicarbonate, which is a flocculant. ... and I was happy to take her/their word for it. PS This point has since amplified by quoting from http://www.marjonceramics.com/pages/Tips/slipmaking.htm Soda ash works to dissolve lignite in clay. ... I've no idea of the details of this mechanism, which sounds more complex than the simple addition of Na+ & CO3-- ions to the liquid. I've only got as far as https://patents.google.com/patent/US3325537A/en The humic acid content of lignite may be solubilized by treatment with alkalies, such as sodium hydroxide and sodium carbonate.

Are you sure?

You may be interested in (but not concerned about): Flambé Magic http://ceramicstoday.glazy.org/articles/flambe_magic.html John Britt investigates the appearance of mysterious crystals in the glaze slop.

You may find this of interest How to Increase or Reduce Crazing in a Glaze How to Adjust Crackle Without Changing the Overall Appearance of the Glaze https://ceramicartsnetwork.org/daily/article/How-to-Increase-or-Reduce-Crazing-in-a-Glaze

I find terra sigillata a confusing term, which is also often unrewarding to use in searches. Some of the issues are described in Terra sigillata https://en.wikipedia.org/wiki/Terra_sigillata The introduction is especially relevant, and I include the first paragraph of it. Terra sigillata is a term with at least three distinct meanings: as a description of medieval medicinal earth; in archaeology, as a general term for some of the fine red Ancient Roman pottery with glossy surface slips made in specific areas of the Roman Empire; and more recently, as a description of a contemporary studio pottery technique supposedly inspired by ancient pottery. Usually roughly translated as 'sealed earth', the meaning of 'terra sigillata' is 'clay bearing little images' (Latin sigilla), not 'clay with a sealed (impervious) surface'. The archaeological term is applied, however, to plain-surfaced pots as well as those decorated with figures in relief.

After some searching I found this paper. Which contains some though-provoking facts and ideas. Recovery and Revival of Attic Vase Decoration Techniques What can they offer archaeological research ? https://tinyurl.com/yecn6xnf Of course the firing (and final effect) of this "black gloss" is different from that of t-sig, but they both start by forming a "colloidal slip" (with or without the use of a deflocculant). p114 Gives the results of X-ray analysis of historic samples and modern "reproductions" - This clearly shows that the originals didn't use phosphorous-based deflocculants (many reproductions used calgol). - The "glaze" contains much lower calcium levels than the body. 118 It is pointed out that - The absence of phosphorous in the glaze implies that plant-ash deflocculants weren't used. - Although not conclusive the potassium levels suggest that potash defloculants weren't used either. p119 Looking for a deflocculant-free method of producing the "colloidal slip". - Using a low-calcium clay (similar to that used in antiquity) they found that they could get acceptable (deflocculated?) results a pH>8.7 and >15°C. - Using the same clay pre-soaked for six months they got similar (not-deflocculated?) results at pH~7 and >15°C. ... this suggests that slip production was a seasonal process, probably during the warmer spring to autumn. ... assisted by over-winter soaking of the clay. The difference in calcium content between different clays is shown in fig2 PS I couldn't help speculating: 1. That the need for low-calcium clay was because that any soluble calcium compounds would act as deflocculants. After all soluble calcium and magnesium compounds are used a deflocculants for glaze slips. 2. Historically they may have found suitable candidate clays simply by paddling round the local clay-pits in late spring, looking for signs of spontaneous colloidal dispersion. Those wishing to find suitable modern commercial clays may need to look at them after a 6-months soak.

Apologies to all, a real senior moment.

If you heat air from 0°C to 1000°C it expands to less than five times its size. [It goes from 273°K to 1273°K and expands to 1273/273 its size.] Water expands by about 1700 times when it turns into steam (over a much shorter temperature range). at's a lot.

You could prevent the mould by adding a biocide. Short lived ones (that require topping up) are hydrogen peroxide and bleach (but bleach may be hard on the hands). A long-lived one is copper carbonate. The Best Way to Eliminate Odor from a Stinky Ceramic Slop or Glaze Bucket - Try to make sure that your waste doesn't get near any aquatic life, especially if you use a long-lived biocide. - I could understand your "wild" clay going mouldy if it has lots of organics in it, but am surprised that shop-bought bentonite does.

Thinks for posting the kiln plate. I hope that the experts will chime in, but I suspect that the max temperature of 1300C means that the element life - when firing to your desired temperature -- is likely to be quite short. For example see the first answer in PS I'm not certain if the figure of 50 firings includes the lower-temperature bisque firings.

You may find this of interest, especially the sections What is Heatwork? and Put Witness Cones in Every Firing. Temperature vs Heatwork – Why We Use Witness Cones https://suemcleodceramics.com/do-you-put-witness-cones-in-every-firing/

I don't know the best place in the UK to get elements, but some of them will probably try to reproduce elements for kilns from defunct manufacturers. For example: Kiln Elements, Repair, Servicing and Support https://www.potterycrafts.co.uk/Products/kiln-elements-servicing Can you post the information from the plate here as well. PS The plate probably looks something like this, partially printed and partially stamped.

Both seem very sound advice (while the elements are worn). So does this mean waiting for appropriate safety glasses? Are we talking about welders glasses for IR & UV protection? In the Studio: Eye Health for Potters https://tinyurl.com/3tcnc8tp PS Or -- in the absence of goggles -- could you measure the rate of temperature rise towards the end and stop the kiln closer to the right heat-work. Although you only have figures for 15C/h, 60C/h & 150C/h. BTW -- assuming you have a controller -- what is the rate of your last ramp?

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing. https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf Did you previously fire by observing the cone drop or just use it as record of the firing?

I haven't heard of this before, do you have a trade-name for them? This explains a/the mechanism, and may be a starting point for some DIY-er . How to Build an ON/OFF Switch for a Permanent Magnet https://interestingengineering.com/video/build-on-off-switch-for-permanent-magnet The ones I saw on the net looked expensive, and possibly overpowered for the application.

Possibly the right wiring diagram, but too blurred for me to read https://eadn-wc04-7751283.nxedge.io/wp-content/uploads/W-EA-820-8-Control-Panel.pdHasf It looks like the lower elements are fed by a power relay, are we certain that it isn't suck? (It might still click.)

Lots of pix at https://tinyurl.com/2rkyb27z ... two particularly lo-tech ones are It would be interesting to know if "fingerdips" work Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\ Apparently suckers can be useful in some circumstances

And I understand that slow-cooling can add to the available effects from some glazes in electric kilns, by giving them more time to develop crystals. Super Cool! Slow Cooling in an Electric Kiln https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln From https://digitalfire.com/picture/bestenazoh Firing for Atmospheric-like Effects in an Electric Kiln https://tinyurl.com/cf7ef94y Fuel-burning kilns tend to be much larger than electric kilns. Because of their size, they usually have a slower heat rise, a soak at the top temperature, and slower cooling cycles. If you want similar results from an electric kiln, especially when firing to cone 6, one of the most important things you can do is emulate the heating and the cooling cycles of larger kilns. This means slowing the temperature gain to about 100°F (38°F) an hour during the last several hours of the firing, soaking the kiln at the top temperature, and then down-firing to slow the cooling cycle. Electric kilns are built with thinner insulation and legs (to allow for air circulation), and they cool very quickly, especially at higher temperatures. If you are seeking buttery, matte surfaces but have trouble achieving them in the electric kiln, it is most likely due to fast cooling. Matte surfaces are usually caused by microcrystal growth during cooling, and, if the cooling cycle is too steep, there isn’t enough time for crystals to develop. In extreme cases, I’ve even seen matte glazes go glossy and transparent. ... note that the 100F(38C) above is "finger trouble", see What is the difference between fahrenheit degrees and degrees fahrenheit? https://digitalfire.com/picture/2704

Can you give a reference? It would be interesting to know why he chose to mix by hand rather than mechanically, sounds a bit hair-shirt to me. You may find parts of this video of interest, especially from about 24m to 31m. It describes the gradual realisation of the nature of the slip used in antiquity for things like Greek red-and-black and "Roman" Samian ware. Terra Sigillata: Lost & Found by Peter Pinnell & Rhonda Willers Long before glazes appeared, clay finishes were widely used. Potters around the Mediterranean developed surfaces employing the unique properties of clay. Once lost, Terra Sigillata was found after 1500 years. This co-lecture provides a history of this surface with an overview of today’s practices. ... of course this doesn't say how it was made. I suspect this is lost in history. I doubt deflocculation would leave identifiable traces, and it the process may well have been a trade/craft secret. Use of deflocculants such as soda/pearl ash would have been possible. I've even seen a suggestion that a good way to find a natural source is to wait a decade or so for a major drought and then explore the dried-up beds in the local lakes. Some of them will consist of ultra-fine particles (sort of a super ball-clay*). Personally I've wondered if waste-water from the production of clay for the bodies could have been a starting point, as it would probably have contained colloidal particles. * Supposedly the change of pH when river water enters a lake favours the precipitation of colloidal particles, potentially leading to a very fine silt. PS The video quotes the historic use of the term "magnetic" - obviously we would now say "electrostatic".