These legacy slip casting molds from Medalta Potteries (made from 80 year old masters). They are difficult and time-consuming to use and produce less than optimal results because they have no top section (this no spare) and require constant filling during cast time. Demolding requires cutting the lip flat (top right). But a lot of time trimming and sponging is needed to round it again, but making the lip even and symmetric is difficult to say the least.

I found a way to make these molds easier to use and better: A 3D printed spare/pouring spout that also defines a rounded rim. It can be glued to the top of the mold with slip. Of course, the PLA print is not absorbent, but this still works because the mold top edge is able to dewater the slip even inside the contoured top it forms. The print also acts as a cutting guide to cleanly cut anway any clay inside the spout section, leaving a clean line inside the lip. And the shrinkage of the clay pulls the pitcher lip away from the print.

This page is dedicated to the skill and intuition of the Plant Technicians who kept the ceramic industry in North America thriving before the 1980s. Before we started clicking buttons to outsource things. They weren’t “role fillers” supplied by HR, they were “believers”. They understood everything in the plant; the equipment, processes, procedures, materials, recipes, kilns and firing. Managers set the pace, but the technicians made the pace possible. It was a time of local knowledge and company loyalty. They weren't temporary consultants or voices on a helpline; they owned and solved the problems. They were also mentors who passed their knowledge down.

These binders hold 40 years of recipes and techniques, kept by Albert E. Holthaus at Modern Art Products and Tierra Royal Potteries. Men like him were a legacy; they were the true "operating system" of a golden age of independence. They ensured the wheels kept turning, the fires kept burning and the quality kept enduring.

This batch-to-formula calculation was done by Albert E. Holthaus at Modern Art Products Company in Kansas City, MO (during the 1960s). Doing this not only seems quaint today, but suppliers put up roadblocks to doing it.

Notice that he took the manufacturer-supplied percentage analysis for each material (bottom) and calculated the unity formula for use in his batch to formula calculation (top). The recipe material weight amounts are missing in the latter; this appears to be his effort to create a documentation page of the recipe on the oxide formula level (this is what mattered to him). It was a time when frit formulas were published by their manufacturers. He also calculated the glaze's chemistry as a percentage analysis, likely to lay a basis to assess it against stated requirements from stain suppliers (certain stains only work when the host glaze chemistry meets a certain profile).

Doing this now is so much simpler. But almost no one actually does! The closest most technicians get to oxide formulas is choosing a frit from a list of ones for which the chemistry given by the manufacturer is only approximate.

This reduction stoneware glaze is producing white streaks on some pieces (left center). The body is a coarse iron stoneware. A magnification is needed to better explain this.

It is 2025, many smartphones now have dedicated macro lenses and can be held as close as a 1 centimeter. They automatically sense placement and switch to using the macro lens. Of course, the phone must be held rock steady and good lighting is essential. If you are a doubter of what they can produce, look at the two magnifications on the right. On the top one, the white streak is clearly visible, floating in a sea of phase-separated glass patterned by earlier-escaping bubbles. The extreme magnification on the bottom right appears to implicate tiny crystals growing in an area where late bubbles have escaped, changing the pattern of phase separation. This doesn’t yet explain the cause, but it is valuable information courtesy of a macro lens.

Place: Vernon, Alabama.
Story: Potter's friend sends a picture of an outcrop of white clay in the ditch near his driveway.
Result: A DIY claybody is born.

This planet is full of accessible clay deposits. Many can be used as-is for stoneware, earthenware and even porcelain. Characterizing this clay is the first step. How plastic is it? What does it look like when fired at different temperatures? Does it contain impurities that need to be sieved out? Does it dry without cracking? Does it work with glazes? Etc.

A journey of clay discovery to a finished piece is one of the most rewarding experiences you can have as a potter. And be more self-reliant. You don’t need special gear, just curiosity, eyes that notice, a few simple tools, and a willingness to experiment and learn to characterize clays. And one more thing: An organized way to keep records of your testing. Think of an insight-live account as a commitment to building experience; it is your memory of everything that worked. And didn't.

Or, more correctly, is this one a clay? The way I found out was to test it myself. That's what I did.

The giveaway of its marine origin is the tiny shells found on the sieve. The Cretaceous Sea once connected the Arctic Ocean with the Gulf of Mexico, covering the great plains of North America. Sedimentation left this deposit of Diatomaceous earth in central Alberta, Canada. This sample contains enough clay that I was able to slurry it up, dewater it on a plaster bat and then prepare SHAB test bars to try it at five temperatures. At cone 10 (bottom right) the porosity is 62%! And the LOI is 32% (others can go as high at 50%). Why? Raw diatomaceous earth contains physically bound interlayer water, it leaves by ~100–300 °C. It also contains structural hydroxyl water (in clay minerals or hydrated silica phases). This “chemical water” burns off between ~400–700 °C. And, organic matter from ancient algae, plants, or soil contamination also burns out between ~300–800 °C (as CO₂ and other gases). Finally, the carbonates (e.g. shells shown here) decompose around 700–900 °C, releasing CO₂. That alone can cause a big weight loss.

Note the test bars under it. Where this bar was sitting there is glassy deposit. What is that? Diatomaceous earth is mostly amorphous silica, but it almost always contains alkali and alkaline-earth impurities and sometimes boron. The latter can literally drain out, as a liquid. However here, the alkalis have volatilized (vaporized) or form alkali-rich fumes. These landed on nearby surfaces to react with the other test bars to form a thin alkali-silicate glass layer (similar to what happens in soda firing).

This is G3948A (similar to the popular Amaco Ancient Copper product). To get this stunning result, it needs to be applied thickly. Therefore, it runs a lot. But the catcher glaze around the bottom of these mugs has stopped the flow. The catcher is a glossy black, G3914A (but Amaco Obsidian would also likely work). I have learned to put it on with the right height (about 2cm) and right thickness, and then apply wax emulsion to prevent the iron red glaze from sticking during dipping. The inside glaze, G2926B, is one I have tested and developed to fit Plainsman clay bodies as a liner.

Watch the G1214Z video to see me convert the G1214M cone 6 clear base into G1214Z cone 6 calcia matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcia matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.

Ceramic glazes, like this GA6-B, are actually just glass. But they are not like bottle glass. The latter is formulated to work well in forming machines (harden quickly), melt and stiffen quickly, have low melt viscosity and resist milkiness and crystallization on solidification. The chemistries to accomplish this have adequate resistance to leaching and adequate durability for a few uses. A stoneware glaze melt needs to be much more viscous (to stay put on vertical surfaces). And, it must have a lower thermal expansion (to match common clay bodies). And, it must resist crystallization much more (since it cools slowly). Fortunately, meeting these needs brings along big benefits: Greater durability, hardness and resistance to leaching. Stoneware glazes and bottle glass share a common trait: They have about the same amount of SiO2. But the similarity ends there, stoneware glazes have:

-High Al2O3. Three to five times more! It is the key oxide for durable glass. And it stiffens the melt (that disqualifies high levels from bottle glass).
-The same fluxes (CaO, MgO, K2O, Na2O). But they distribute very differently (half the CaO, half to one third the KNaO, much more MgO). Other fluxes like SrO, Li2O are also common.
-Low KNaO (which they call R2O). In glazes, it produces crazing, 5% is a typical maximum. But bottle glass can have double or triple that (the high thermal expansion is not an issue, and its cheap source materials supply lots of melting power).
-B2O3 melter. It is expensive but can be justified because the glaze is just a thin layer. Glazes at the low end of the stoneware range have 5% or more boron.

Far right: A glass bottle. Left: Small test bottles made from dark and light burning stonewares. Third: A production ceramic bottle. Notice how much the dark body darkens the GA6-B glaze.

I have seven open side-by-side. There are hundreds of them, and all are well-documented with test results and photos. There are glazes, engobes, bodies, materials and special-purpose recipes. All of them are ones that have been shared over the past decade from our Insight-live.com account. These are great to open beside recipes you are evaluating or testing, it can be a real eye opener to see the chemistries and recipes compared.