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The clay aging myth: Prove it wrong by slurry mixing

Clay aging: It is a myth

Do you really need to age clay when you make your own? No. In ancient Japan they did not have power blenders and propeller mixers. We do. To illustrate: I just sieved out the +80 mesh and +200 mesh particles from this raw clay (from one of our stockpiles) and then propeller-mixed it as a slurry. That wetted the particles very well and made it easy to sieve. Then I poured the slurry on to a plaster table and thirty minutes later it was ready-to-use. Slurry mixing is just as good as deairing in a pugmill. No wait! Particles wet even better. The plasticity of this clay is wonderful, and, it will not get any better with aging. Ancient Japanese potters used non-plastic, coarse particled clays so they needed to squeeze every last bit of plasticity out of them. Today, fine particled plastic clay materials are readily available. And we have something else the ancients did not: Micro-fine bentonite. A few percent of that and any clay can be made super-plastic (provided you have a good mixer to wet and separate all the particles to release their full power).

Context: Bentonite, Mix the slurry using.., Propeller Mixer

Tuesday 28th May 2024

The difference propeller-mixing makes in a titanium glaze

Titanium opacifies if mixed well

The glaze has 5% added titanium dioxide. These were fired at cone 6. The titanium in the one on the left remained agglomerated, it did not disperse in the slurry during hand mixing (the agglomerates can be seen as white particles floating in the glass). On high-speed propller-mixing the effect on the right was produced! Blender mixing would be even better. This incredible difference occurs because the mixer is able to break up the titanium agglomerates, dispersing and wetting all the surfaces of the incredibly tiny particles. In this state they do their magic during the firing, opacifying and variegating the otherwise transparent base matte glaze. Would sieving be as good? Only partially, the particles are 200+ times smaller than the openings in an 80-mesh sieve, high energy input is needed to separate them.

Context: Agglomeration, Propeller Mixer

Tuesday 28th May 2024

77 million-year-old mugs cataloged and exhibited

77 million-year-old coffee mugs

These mugs are made from clay mined from the Whitemud formation in southern Saskatchewan. Those layers date from the late cretaceous, the worlds largest T.Rex was found nearby. The errosive forces of nature during that period did a marvellous job of purifying and depositing this beautiful high temperature stoneware clay. It is basically complete right out of the ground, with just the right amount of feldspar and silica added to a ball clay and kaolin base. The glaze on these is G2571A bamboo, the engobe under it is L3954N black. They were fired at cone 10R. These are examples of mugs I make and catalog, each has its own pedigree web page that celebrates every detail about the materials and process used to make it (e.g. here is a pedigree page, it links to the mug's store URL). However please don't attempt to buy any, I am using the store as more of a gallery for now.

Context: Clay in dinosaur country.., Mel Noble at Plainsman.., The world's largest T..

Monday 27th May 2024

A black engobe transforms the floating blue glaze over it

Floating blue over black engobe

This is M340 stoneware glaze fired to cone 6 using the C6DHSC schedule. The L3954B engobe fires deep black (it has 10% Mason 6600 black stain). The engobe was applied by pouring and dipping at leather hard stage (inside and partway down the outside). After bisque firing the piece was glazed inside using the base GA6-B Alberta Slip amber base. The outside glaze is Alberta Slip Rutile Blue GA6-C (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers).

Context: L3954B, GR6-M, G2826R, FLB

Sunday 26th May 2024

What should the consistency of CMC gum solution be?

Gum solution consistency

This is CMC gum 35g/liter gum solution after it has been thoroughly hot-mixed and cooled to about 30C. As it cools further and sits it thins. Gum solutions can have a higher CMC content, up to double this, but they are more difficult to use.

Most often, gum solution is intended to augment the water when batching a recipe. If added to an already-mixed glaze it thins it - so an equal amount or more water should first be removed. For example, consider converting a dipping glaze to a brushing glaze: Adding 1.5% CMC gum, via gum solution, to a gallon of glaze would also add almost a liter of water (it would become useless and settle out). This being said, a gelling agent like VeeGum can also be added to create a low specific gravity slurry, like many commercial bottled glazes. This enables fine control of thickness since multiple layers must be applied.

Context: CMC Gum, Control gel using Veegum..

Tuesday 21st May 2024

This amazing difference 45 micron silica can make

A glaze stops crazing with 45 micron silica

The only difference between these two cone 6 glazes is the silica. Both are the G2926B recipe, both were thickly applied and fired in the same kiln. The left one employs the 90 micron (or 200 mesh) grade silica and the right one uses 45 micron (or 325 mesh). These test tiles are about 6 months old. There was no crazing out of the kiln. The porcelain recipe is 25% silica, 25% nepheline and the remainder kaolin and bentonite. It appears the finer particle size silica is dissolving in the melt much better, this narrows the difference between calculated and actual behavior, especially relating to coefficient of thermal expansion. While this grade is better in glazes it is not better in bodies, they most often depend on the thermal expansion increasing effects of the larger particles in the 200 mesh grade.

A reader noted that it is also a matter of the reaction between glaze and body. The original glaze having coarser silica would have smelt and reacted with the body more, the extra dissolution sourcing Na2O - thus increasing the COE of the glaze. Conversely, when the finer silica dissolves it increases melt viscosity thus reducing reaction with the body.

Context: Silica, We thought we were.., The difference between Silica.., Body glaze Interface

Thursday 16th May 2024

An impossible spout is possible by 3D printing

3D printed mold spout

This mold is for a Medalta Potteries ball pitcher having a closed top with a teardrop-shaped spout (lower right). This plaster mould does not need a spare - 3D printing makes it possible to create a pour spout that inserts perfectly into the angled hole. The size of the pour spout reservoir and the degree of insert can tuned so that when the level drops to the bottom it is ready to pour and the hole is perfectly formed.

Context: Pour Spout

Wednesday 15th May 2024

Why are rutile blue glazes susceptible to this blistering problem?

Rutile glaze has blistered

This blistering problem is common in rutile blue glazes, especially high-temperature - this is not saleable. The reason relates to what it takes to create this kind of vibrant variegated aesthetic: Melting the crap out of the glaze and cooling it just right. This particular one is being fired to cone 11 down to get enough melt fluidity to make it crystallize and phase separate. It seems logical that if the glaze is melting so well it should be able to heal any bubbles that form and break (these are more than usual because the body is being overfired and generating gases). However, the fluidity comes with surface tension that can hold the bubbles intact. Each of these holes in the glaze is a product of that - plus another factor: Cooldown is rapid enough that the melt is not sufficiently fluid to heal after bubble breakage. The potter has been using this glaze for many years with success, but a small change in process or materials has occurred to push it past a tipping point. Solutions? A drop and hold firing. Add a flux (e.g. a little lithium or a frit) to make it melt fluid at cone 10R (where the body generates less gasses of decomposition). Replace any high LOI materials in the glaze itself with other materials to source the same oxides.

Context: Rutile Blue Glazes, Glaze Blisters

Tuesday 14th May 2024

Why 3D design and printing is a better way to make slip casting molds

3D printed plaster mold master

I have not made slip casting molds for years because I dread the process, the mess, all the supplies and tools. I am not a mold-making expert either, but I found a way to do it that is fun, rewarding and effective.
-I am wasting less plaster (it is not a green material). And PLA filament is corn starch or sugar cane. And I am not using rubber.
-I spend most time on design, pouring the plaster takes minutes.
-Many fewer tools are needed, the process is less messy.
-No natches make sanding of flat mating faces possible (for better seams than I've ever had).
-No spare is needed, the 3D-printed pour spouts works better.
-More shapes are possible.
-My molds aren't right until at least version 3. 3D makes do-overs or changes in design as easy as a reprint and plaster pour. I can make a mold just to test an idea!

Context: Beer Bottle Master Mold..

Sunday 5th May 2024

Drying cracks in bricks - but no data to determine best response

Bricks cracking during drying

These bricks were being extruded in India and the plant was suffering drying cracks. A consultant recommended a high percentage addition of lignosulphonate, at a cost of $800/ton, as a solution. But before adding such a large expense, some obvious changes seemed in order first. The technician knew the plasticity index of the clay (a measurement used for soils) but he did not have records of its drying shrinkage, water permeability, drying strength or drying performance - when problems like this arise such data provides direction and help answer questions. For example, is cracking happening because of lack of drying strength or plasticity or because drying shrinkage is too high. The splitting along the corner of the extrusion is a clue that plasticity could be lacking - that could be solved by a small bentonite addition or reduction in grog. If permeability is low an increase in grog might be needed (if the pugmill can still extrude slugs with a smooth edge and corner). Notice the cracks that start from those splits (lower left). But also notice how the top edge has shrunk while the center section has not. That indicates the drying process is not tuned to subject all surfaces to equal airflow (sure enough, these are being dried outside in the sun and wind). Another factor is cross-section: The round holes create variations in thickness that exceed 300%, square holes with rounded corners would be better. Given the location, economic realities and past success this one change might be enough to make a big difference.

Context: Simple Physical Testing of.., Bricks and tiles are.., Clay lab report Is.., Physical Testing, Cracking of Clays During..

Tuesday 30th April 2024

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