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2019 Jiggering-Casting Project of Medalta 66 Mug
A cereal bowl jigger mold made using 3D printing
Beer Bottle Master Mold via 3D Printing
Better porosity for Brown Sugar Savers
Build a kiln monitoring device
Celebration Project
Coffee Mug Slip Casting Mold via 3D Printing
Comparing the Melt Fluidity of 16 Frits
Cookie Cutting clay with 3D printed cutters
Evaluating a clay's suitability for use in pottery
Make a mold for 4-gallon stackable calciners
Make Your Own Pyrometric Cones
Make your own sieve shaker to process ceramic slurries
Making a high quality ceramic tile
Making a Plaster Table
Making Bricks
Making our own kiln posts using a hand extruder
Medalta Ball Pitcher Slip Casting Mold via 3D Printing
Medalta Jug Master Mold Development
Mold Natches
Mother Nature's Porcelain - Plainsman 3B
Mug Handle Casting
Nursery plant pot mold via 3D printing
Pie-Crust Mug-Making Method
Plainsman 3D, Mother Nature's Porcelain/Stoneware
Project to Document a Shimpo Jiggering Attachment
Roll, Cut, Pull, Attach Handle-making Method
Slurry Mixing and Dewatering Your Own Clay Body
Testing a New Load of EP Kaolin
Using milk as a glaze

Yixing Teapots

The Yixing TeapotCraftsman videos on Instagram make the craft seem almost magic. Apart from their obvious skill in hand-building perfectly round pieces, they seem to be exceeding process limits in multiple ways:

-The clay appears highly plastic yet is not.
-Craftsmen flatten the clay with a mallet, instead of rolling it, yet it does not stick to the board.
-Sections are simply glued with slip and they hold.
-The clay burnishes yet is not smooth.
-Fired ware is smooth yet the soft clay appears sandy.
-The fired surface is glossy yet there is no glaze.

A Yixing teapot, perfectly round yet made by hand!

It is said that their clay is “well-suited to making teapots in the Yixing fashion”. This is the first clue to understanding how they do it: “Yixing” is as much about technique as the clay itself (of which many types are available). They are adapting their methods to what that clay demands. Here is the first surprising fact: The raw lumps do not just slake with water like typical clays found in nature. Pieces of the ore are typically left out in the sun for a few months, after which they crumble. It then gets pulverized into the desired mesh sizes. For the finer ores, they don’t allow the ore to get too wet when left outside, otherwise the finer particles can get washed away. Note what that means: Yixing clay is a mix of very fine particles with coarse ones.

Working Properties

Plasticity, stiffness: As noted, craftsmen/women appear to handbuild with soft plastic slabs by simply gluing them with slip. But, note some things. First, they are not “hand building”, they are assembling carefully cut polygons. Second, the clay is not plastic in the traditional sense (e.g. it cannot be thrown on a wheel). Note that they must shape it carefully using a paddle. That being said, the wet strength of Yixing clays is impressive, helping prevent curved forms from collapsing (likely due to a stiff plastic base having coarser-grained clay particles).

Third, the clay is not soft, it is stiff (that's why they have to flatten it with a mallet). Notice during flattening that splits appear on the edges (not an issue since they cut shapes within the slab). Yet the splits are small enough to make it obvious this is not just stiff clay. Very fine and highly plastic clay particles are holding the coarse ones together. Curiously, they make the degree of splitting a visual measure of the right stiffness/plasticity combination.

Texture: Yixing clay is said to be relatively coarse-grained. This statement is misleading. First, it is actually fine-grained but having a significant percentage of coarse grains within. Second, these are not “grains” in the traditional sense, being sand or grog particles. Rather, they are just coarse clay, the same clay. They break down into smaller pieces if enough pressure is applied (making burnishing possible). For this reason, the burnishing persists through firing because the coarse particles shrink with the rest of the clay. The behavior is not perfect, producing the visual graininess in the fired surface (but without feeling that way). The softness of the particles enables another property: The artists can break the particles down to just the right degree when making slip (using a stick on a stone board).

Rewetability: Yixing clay is not suitable for throwing because it is sensitive to water, being rewettable but also disintegrating if too much water is added. The permeability enables it to readily reabsorb moisture in a plaster damp box. This is crucial for multi-session handbuilding, where pieces need to be re-moisturized overnight to continue working. This property testifies to how many coarse particles are needed to impart this permeability.

Fired Surface and Appearance

The typical unglazed fired surface is characterized by a distinctive sheen, smooth to the touch. To achieve this, each type of Yixing clay has a firing procedure that must be discovered by the potter. The degree of sheen possible is different with each, some will bloat before high gloss can be achieved. Teacups require the most dense ceramic, teapots can tolerate and actually need some porosity.

Color: Deep red varieties of Yixing can highly vitreous. This is very unusual, and not fully understood, since iron-bearing bodies almost always fire dark brown. Darker browns and black coloration is more a matter of the mix of metal oxides present.

Bloating: Firing is carefully scheduled and maxed to the temperature just below where bloating or blistering occurs (some pieces even have bloating). It is not fully understood how the ware can have such good fired strength since this state produces brittle ware with other raw clay materials.

Warping: Since fired maturity is the key property, pieces must be constructed in shapes that are inherently as stable as possible. Slight warping can be tolerated.

Burnishing: Why does it burnish well if it has so many coarse particles? As already noted, Yixing clay is filled with coarse clay, which will break down to smaller pieces if enough pressure is applied. Malleting to flatting the slabs appears to leave the particles in tact.

Duplicating Yixing

As noted, making these teapots is as much about the procedure as the materials. Much determination is required to learn the techniques and even more is needed do the dozens of testing rounds likely required to similuate the clays. This is not at all about chemical ratios, you don’t need to know anything about that. This is about the physics of the clays. As yet we have not been able to take the first step, characterizing some real Yixing clays. Data from the SHAB test, DFAC test and SIEV test would be super interesting. And it would be even more interesting to see variation between the different Yixing types.

Our modern programmable electric kilns are a real enabler to being able to try different firing schedules, especially just about and below where bloating occurs. Also, the range of already-refined commercial materials available to use now is likely well beyond what any ancient Yixing craftsman had. Another key enabler we have now is detailed record keeping software (e.g. in an account at insight-live.com) and the test data it generates to be able to compare physical properties of multiple tests.

The obvious and authentic way would be to find an iron-bearing weathered shale (like those used in brick manufacture) and develop a way to process it to a mix of fines and coarses (the latter having a size distribution and not being too hard). Hao Tong Yan is in a good position here. She knows what the working consistency should feel like, thus what direction the next test should be. The author has worked with shale materials from Montana and one from the Niagara escarpment that might be processable. To achieve plastic strength in a non-plastic body already having the right particle size mix, an addition of bentonite and/or 200 mesh ball clay could be employed. Another strategy is to blend two or more different local materials, such as a fine silty clay with a coarser less plastic, rewettable product.

The second option is to use the ceramic materials available from your local supplier. Using this approach it seems wise to target the working properties as the priority, and accept whatever other firing properties they produce. Once that is achieved one could fine-tune colour by adding stain or iron oxide. If the body turns out far too refractory, an addition of a frit, as is done with my Zero4 porcelain, would mature it.

The percentage, size ditribution and nature of the coarser particles is the most difficult part. Hao is simulating them by making a low fire grog from the very clay being used. When just slightly sintered the particles "have give" like the genuine Yixing clay. It is just a matter of experimentation to discover the right sintering temperature. Of coarse, a range of sieves are needed to be able to process the particulate to the right sizes.

Achieving sheen and color: Of course, this can be achieved by a stained spray-on layer, best done at the bisque stage, that melts to the desired degree. For red color from the body alone, it is more complicated. Iron-bearing vitreous bodies almost always fire dark brown (ed is achieved by making them less vitreous, meaning the sheen is lost). This is the case with Redart, the most available red burning clay. It seems possible that redness could be achieved by mixing in a body stain. Another option is the application of a red burning engobe at the leather-hard stage.

Hao’s current testing employs various North American materials. For example: 50% Redart, 30% OM-4, 10% silica, and 10% K-spar, firing to near zero vitrification around cone 5.5.