Some people use materials directly from the earth to make pottery. A high volume example of this is the brick maker down the road. (If you don’t have clay or shale where you live, there’s no brick maker down the road.)
Brick makers often extract clay using the opencast method. That means they usually don’t tunnel for the clay. They carefully remove overburden—dirt, weeds, trees, old cars, and whatever is on top—leaving a clean clay or shale face. They then extract the clay or shale and deliver it to the factory.
At the factory, it is crushed or ground as required, mixed with water, and the clay is extruded by means of an auger into a very long rectangular cross section that is cut into bricks as it progresses along the conveyor belt.
Today, everything is automatic in most plants, including loading and unloading bricks into and out of the kiln. (See Ceramics: industrial processing and testing, John T. Jones and MF Berard, Iowa State University Press)
In other words, brick making has a composition of only one component (clay or shale). Now there is always an exception. Clays and shales have some sulfate content. This is not OK. It may have blisters, foam, or some other nasty flaw.
What do they do?
They add barium carbonate or some other chemical to bind the sulfates together because barium forms insoluble sulfates. These materials are not pre-mixed into the clay. They are simply sprinkled on the clay and the extruder makes a sufficient mix.
I will tell you a secret. Some other manufacturers may add barium sulfate, such as sanitary ware manufacturers that make their bathroom fixtures. Some potters also add it.
Vanadium compounds cause foam in bricks that would otherwise be quite white.
Defects caused by raw materials are often related to impurities or particle size. The defect can appear at any time during the process. Here are some examples of defects caused by impurities:
impurity defects
Lignite
Brown coal is common in many clays. It can be hard anthracite or softer bituminous coal or lignite which is softer than the first two.
Screening can reduce lignite from slurries.
Sodium carbonate can react with the soft brown coal and convert it into a useful colloid that will keep the suspension dispersed.
In other words, some lignite may be beneficial in slip casting and other slurry operations. But hard anthracite and other varieties of coal are not good. They can cause blisters and pitting during bisk and gloss cooking operations.
Clay companies have some control over the size and amount of lignite in their clays (often through blending).
You can run a screen analysis on the material and see how much brown coal is left on the screens. If it’s higher than previous shipments, call the clay company and say these words: “What’s up with all this lignite!”
That should get your attention. I once said, “What is all this plastic?” (The filter bags had melted in the air flotation equipment.)
Be sure to blame them for all the defects generated over the next six months. (Hey, you think I’m kidding! Do it in the next couple of months or when that batch of clay runs out.)
Sand
We had a recurring blistering problem on the fine china I worked on once and for years no one could figure out what was causing it.
He was new to the company but not inexperienced in solving material problems.
I called each of our providers and told them we had blisters and it was their fault.
The representative of a kaolin company asked: “What is the grain?”
I wasn’t sure what he was talking about because I hadn’t used that particular clay before. Instead of telling him I didn’t know what he was talking about, I said, “Wait a minute.”
I asked the quality control manager, “What is the value of those things?”
He had a questioning look on his face despite his 30 years of service. She pulled out the spec sheet from the last shipment, found that there was such a thing as “grain,” and gave me a number.
The number represented the amount of coarse material that appeared during a screen analysis of the clay. For this particular clay, the sand was in the form of mica.
Mica is not good!
Hearing the number, the representative said, “It’s too loud! I’ll call the mine!”
We established a control standard and never had that problem again.
I paid my first year’s salary in five minutes a few days after joining the company.
Iron and manganese compounds, silicon carbide, soybeans and salts
Clay makers sometimes ship clays to storage areas by rail, ship, or barge. Storage of raw materials at docks is always dangerous for contamination. The reason is that these facilities ship iron ore, ferro-silicon, silicon carbide and do not take great care in cleaning a shed of ferro-silicon or other contaminants before loading it with a cargo of clay. (We switched from bulk shipments to Super Sack®).
Some of these materials give off oxygen when they decompose. That occurs at some characteristic temperature.
We use the same British china clay at two different plants.
On one floor we had blisters on our decorated tableware, the worst possible condition.
The other factory that used the same clay did not. Why? The bisk temperature was much higher in the unaffected plant. The contaminant, a manganese compound, decomposed prior to glazing and decoration.
The affected plant had lower bisk and gloss temperatures. The material did not fully decompose at these temperatures and bubbles continued to appear at decorating temperatures.
The problem was complicated by the fact that we had to prove that the supplier caused a large loss that his insurance company would have to pay.
I will not go into details because they proved that we were right and we paid.
The reason for the refund was that I had asked them to never send us material from a certain well (mine). He had caused the company losses for years and he wanted no more.
They shipped from that pit instead of sending us the processed clay I specified.
Impurity identification can be tricky without a scanning electron microscope (SEM).
Heavy liquids can be used, but that’s a hassle. Microscopes are good for those who know how to use them. You can observe contaminants from heavy liquid separation.
Sometimes you have to use an outside lab if you don’t have the right equipment.
To isolate a contaminant before shipping samples, remove clays and filter non-clays.
To elutriate a clay, take about ten pounds of clay and keep washing it until only the contaminant remains. Put the contaminant on your body or glaze and reproduce the defect you are seeing in production.
Chloride can be detected in the salts by washing the clay with deionized water and testing with a soluble silver nitrate solution. A milky precipitate is silver chloride. The salts cause blisters!
We once had soybeans in one of the plants I worked at. Clay collected beans from a hopper car. They poke big holes in your produce, but are easy to leak if you have a mesh in your operation, which we didn’t!
There is a funny story that goes with soybeans but I don’t have space to tell it here. I’ll save it for another article.
particle size
One of the most serious formulation mistakes is not controlling the size of the particles in the batch recipe.
For example, if you use too much of a certain clay that has a very fine particle size, you will have problems. Let me say this, you must choose the correct particle size distribution for your process.
Slip casting requires a coarser particle size distribution than a plastic forming process for the same formulation. What does all this nonsense mean? You should use coarser grained clay(s) in your pouring process. If you don’t, you’ll still be trying to cast a piece when the next millennium rolls around.
If you want to be in control of your casting process, use a coarse-grained kaolin plus a fine-grained kaolin and a coarse-grained ball clay plus a fine-grained ball clay. Changing the ratio of fine to coarse clays will give you control. You must keep the full amount of kaolin and the full amount of ball clay to preserve the color of the product.
Look at the particle size distribution of the clays you are using in your process. This information must be provided to you with each shipment.
Look at the part of the particle size that says