By JDZ-Ceramic
Section: Clay Bodies, Subsection: Formulation
Description
Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them
Article
Here are a couple of starting recipes for casting stoneware bodies. It is assumed, of course, that you might need to make adjustments. Remember that property adjustments are always on a trade-off basis. If you want a super white body, for example, you are going to have to give up some working properties.
- Maturity: For greater vitrification increase the feldspar by 5% and reduce the clay by the same and test (vice versa for less vitreous).Â
- Thermal expansion: Increase the silica to kill crazing, reduce to eliminate shivering (remember that ball clay contains significant quartz).
- Casting rate: Choose a larger particle size kaolin or ball clay for faster casting (there are large particle ball clays promoted for use in casting, note that these might be higher in iron). Don’t struggle with a slow casting slurry, it is not worth it.
- Whiteness: The ball clay and kolin are both clays. Minimize ball clay in favor of white burning kaolins to whiten, but watch for losses in dry strength or ability of ware to pull away from the mold. There are many different kaolins with an array of advantages and disadvantages. The ones that are good for plastic porcelains will often impede working properties of a casting slip because of their fine particle size. For optimal whiteness remove the ball clay entirely and use all kaolin.
- Translucency: Use clay materials having minimal titanium content and use more feldspar for better melting.
Once you get a feel for why each of the materials is in a porcelain or stoneware body you will find that balancing all of the factors involved will lead you toward only one possible recipe. For example, you need enough silica so glazes will not craze and enough feldspar to vitrify the body. That leaves the remainder for clay which you can mix and match for your needs. If two researchers are given a clear description of the properties needed in a porcelain, the way it will be fired and the set of materials to work with they will come up with the same recipe. Read more »
By JDZ-Ceramic
Section: Clay Bodies, Subsection: Formulation
Description
The classic white ball clay talc casting and modelling recipe has been used for many years. It is a dream to use as long as you are aware of the problems and risks.
Article
When you buy a low temperature white burning body from a ceramic supplier you are buying what the industry calls a ‘talc body’. Talc bodies are basically 50% ball clay and 50% talc (variations are discussed below). If your supplier says the recipe is a secret, let it pass, everyone is entitled to their beliefs. On the other hand, some suppliers have gone through a lot of effort to choose the best ball clay and talc available in their area and they might be putting in a small amount of other things. The ball clay in this body gives it great dry strength for handling and the talc imparts the good casting, drying and glaze fit properties (more on that in a minute). The use of this type of body started in the hobby casting industry and it became a standard on which the prepared glaze industry could rely. They created all of their products to fit this type of body. This standardization was a big reason for the success of what the commercial ceramic manufacturing industry would regard as an unorthodox and problematic recipe. However hobby cast ware has even thicknesses and is fired relatively slowly in top loading kilns so the body does not suffer process related failures that large industrial users would encounter (firing cracks, glaze crazing). Almost anyone can cast it successfully and very difficult shapes can be made because of its high wet and drying strength. It has both a fast casting rate and good strength (qualities not normally found together). Talc bodies are also very stable when fired to cone 05-06, they do not warp, even on thin pieces. So again, hobbyists with no knowledge of ceramic manufacture can make complex overhung shapes and never even think about warping issues. Another factor is that talc bodies often fire amazingly white compared to stonewares and thus bright colored glazes work well on them.
Manufacturers of modeling, throwing and sculpture bodies figured out that this 50:50 talc:ball clay recipe could be adapted to plastic forming by simply adding a little bentonite to improve plasticity. Such bodies have the best throwing properties, for example, of any clay available. The beauty of this adaptation is that the whole world of prepared glazes is then available to potters and sculptors.
The Down Side
There has to be some downside to this. There is.
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By JDZ-Ceramic
Section: Clay Bodies, Subsection: Formulation
Description
Understanding the functions of each of the major materials in a high temperature porcelain gives you the ability to tune their amounts and choose brand names to make the porcelain you want.
Article
Incredible strides in porcelain ware production and firing equipment have occurred in the last couple of decades. Robotics and computer controllers have revolutionized the whole ceramics industry. However, clay bodies themselves have tended to resist change. Perhaps it is much easier to understand and troubleshoot a machine than a clay body. Understanding the dynamics of powder, slurry, and wet materials processing, forming, drying, and firing is not easy. Let’s try to take at least some of the mystery out of pottery porcelain formulation.
A pottery porcelain is actually just a vitrified clay body with low Fe2O3 contamination. A general porcelain recipe is fairly easy to derive. The initial thought process goes like this:
- The only common low-iron clay is kaolin. Kaolins normally have low plasticity, so some plastic ball clay having minimal iron may be needed.
- These clays are refractory, so fluxes are necessary to make them mature at a lower temperature. Feldspar is effective and it is inexpensive.
- Adding silica reduces body expansion making it easier to fit glazes and it acts as a cheap filler and firing stabilizer.
The “Universal 25 Porcelain” recipe is a product of this type of reasoning. It is made from 25% each of ball clay, kaolin, feldspar, and silica or more simply 50% clay and 25% each of silica and feldspar. Thousands of potters and companies use this as is or alter it to accommodate specific materials or circumstances. Let’s use this recipe as an example and a basis from which to learn how to optimize a porcelain to your own circumstances. Also, I will propose an approach to creating a porcelain from scratch.
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By JDZ-Ceramic
Section: Glazes, Subsection: Color
Description
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
Article
Stains are fired blends of metal and ceramic oxides that have been reground into a fine powder. Stains containing otherwise toxic oxides can be employed without significant dangers. This is the first aspect of something that stains have that coloring oxides don’t: stability. A second aspect of stability is that stains produce much more consistent and repeatable color than using raw oxide colors.
Stains are most popular at lower temperatures where colors tend to be brighter. However most stains can be used right up to high fire. Premixed low fire glazes are typically made by blending stains and commercial frits and other than following firing instructions, users of these products give little thought to the technical challenges that were overcome to produce them. This is a third key advantage of stains: the ability to target a specific color. Many ceramic color shades (i.e. reds) are difficult to achieve and beyond the abilities of end users.
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By JDZ-Ceramic
Digital kilns with excellent pyrometers allow studio potters to achieve remarkable glaze effects through planned and controlled cooling and ‘holding’ cycles.
But how are these points and times decided upon? And what, exactly takes place? In a research-level glass laboratory we could do many experiments, construct the relevant phase diagrams, extrapolate from them to determine our holds.
Unlike metals, ceramics do not come to equilibrium in a time span corresponding to human activity. While physical chemists have techniques for dealing with this, potters do not! All we can do is experiment within the framework provided by a few facts, such as that nothing happens below the temperature at which the glaze is a solid. We test between the temperature of the desired cone, and the point where the glaze is a solid.
A look at those few facts.
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By JDZ-Ceramic
Section: Glazes, Subsection: Base Glazes
Description
This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.
Article
This page is a companion to the article on the cone 6 glossy G1214W base glaze recipe (which is in turn a derivative of the G1214M). Important information is available on the G1214W article and we suggest you study that page first before reading this one.
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By JDZ-Ceramic
Section: Glazes, Subsection: Chemistry
Description
This discussion was initiated by a question in July 1998 on Clayart as to whether a knowledge of eutectics would help produce a better glaze.
Article
This discussion was initiated by a question in July 1998 on Clayart as to whether a knowledge of eutectics would help produce a better glaze. David Hewitt provided some helpful information and Ron Roy made a thought provoking statement:
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By JDZ-Ceramic
I revere, recycle, reuse and renew laguna B-mix white stoneware cone 10, for handle pulling clay. The clay and it’s plasticity is the most important factor for the handles to maintain integrity through all steps: pulling, shaping, drying and attaching to pottery. The clay handle has to be pulled perfectly with no flaws in it’s composition or delivery. Any problems will show up in the bisque and glaze high firings and the clay handles will not sustain their shape or integrity.
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By JDZ-Ceramic
Section: Glazes, Subsection: Adjustment, Adaptation
DescriptionSomeone is having a problem with a cone 6 glaze going glossy and crystallizing, this article rationalizes the problem in terms of chemistry
Article
Following is a letter we got from someone who was wondering whether INSIGHT software could help fix a glaze problem:
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By JDZ-Ceramic
Section: Glazes, Subsection: Adjustment, Adaptation
Description
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Article
There are many complex and expensive instruments designed to observe and measure the goings-on in firing kilns. Generally this type of equipment is expensive and measures absolute physical properties that can be quantified easily. However glaze melt flow is like clay plasticity, it is more subjective and not so easy to quantify. It is best measured comparatively, that is, one specimen directly compared with another. Fortunately such tests can be done using inexpensive methods and devices.
I would like to submit a general purpose testing method for many glaze melt properties that is both inexpensive and easy to use. So many factors related to the melting, solidification and physical properties and defects of fired glaze surfaces are related to melt viscosity. Thus a test that provides information about this has the potential of being very valuable.
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