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

Yixing craftswomen at work

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. How, on closer scrutiny, this seems to be a triumph of how working within constraints can produce excellence. I have crossed paths with Hao-Tong Yan, who can give us a look inside. This potter is simply trying to do the craft in North America. No one is advocating counterfeiting Yixing ware; we are just putting our heads together to characterize the clay and better understand the process. Perhaps a new genre will credit its roots to Yixing ware.

Consider the Magic:

-The ore is rocks; how can they become a workable clay body?
-The clay appears highly plastic yet is not.
-The soft clay has an impossibly low water content.
-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.
-The clay appears super dense yet does not have porosity (to preserve tea taste).
-The ware shrinks far more than typical pottery, yet does not warp.

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 for use pure or by intermixing). They are adapting their methods to what that clay demands. Here is the first surprising fact: For some types, the raw lumps look like rocks, they do not just slake with water like typical clay ores. That being said, all clays are a product of a weathering process of parent rocks (in geological time). Yixing clays are subjected to an accelerated weathering process measured in years. They are typically left out in the sun and rain for years (or months for some, lest the fines are washed away), during which they crumble (and helped along with a hammer). Substandard and contaminant lumps are removed during this period. It then gets pulverized and ground into the desired mesh sizes, water added and stored in cellars for aging (for years, even decades). Note what that means: Yixing clay is a mix of very fine particles (suggesting siltstone) with coarse ones (suggesting shale).

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/fitting carefully cut polygons. Second, the clay is not plastic in the traditional sense (e.g. it cannot be thrown on a wheel), they must shape it carefully using a paddle. The stiffness imparts impressive wet strength, helping prevent curved forms from collapsing.

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 has 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. Yixing clay contains much less water than standard clay for the same level of workability. One sample tested had 15% moisture content, and was still too wet to use (drying it to 10-12% moisture was needed). Typical pottery clays are leather hard at this level!

Fired Surface and Appearance

A Yixing teapot, unglazed yet functional!

IThe most coveted unglazed fired surface is characterized by a distinctive sheen, smooth to the touch. To achieve the best of each type of Yixing clay, the firing procedure 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. The fired shrinkage, an indicator of density, varies widely.

Color: Deep red varieties of Yixing can be 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. A typical pottery would be mortified at the amount of shrinkage that occurs during firing; it can be an incredible 20%+. One can only imagine how difficult it is to prevent warping.

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 flatten the slabs appears to leave the particles in tact.

Duplicating Yixing

Yixing perfection, a high standard!

Shrinking ore supplies and rising prices lead to talk about “false Yixing” containing “questionable chemicals”. But no chemicals are needed, just minerals having similar enough “physics” and modifying techniques to adapt to them. 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 to do the dozens of testing rounds likely required to simulate the clays. This is not at all about chemical ratios; you don’t need to know anything about that.

As yet we have not been able to characterize 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. Imagine if they had Veegum and deairing pugmills instead of 10 years of aging. Another key enabler we have now is detailed record-keeping software (e.g. in an account at insight-live.com) and ways to search and compare test data generated 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 is in a good position here to what the working consistency should feel like, thus what direction the next test should be. The author has worked with shale and siltstone 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. Of course, it would not be possible to achieve plasticity at the low water content levels Yixing can do.

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 distribution and nature of the coarser particles is the most difficult part. Hao-Tong 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 course, a range of sieves are needed to be able to process the particulate to the right sizes.

Yixing in North America!

IAchieving 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 (red 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-Tong’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.