Ceramic and Pottery Defects 2: Defects from Raw Materials and Batching Errors

John T Jones, Ph.D.
 


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Some folks use materials right out of the ground to make ceramics. A high-volume example of this is the brick manufacturer up the road. (If you don’t have clay or shale where you live, there is no brick manufacturer up the road. )

Brick manufacturers usually mine clay by the open pit method. That means that they usually don’t tunnel for the clay. They carefully remove the overburden (the dirt, weeds, trees, old cars, and what-have-you on top) leaving a clean clay or shale face. Then they mine the clay or shale and deliver it to the factory.

At the factory, it is crushed or ground as required, water is mixed in, and the clay is extruded by means of an auger into a very long rectangular cross section that is cut into brick as it moves along the conveyer belt.

Now days, everything is automatic in most plants including the loading and unloading of bricks into and out of the kiln. (See Ceramics: Industrial Processing & Testing, John T. Jones & M. F. Berard, Iowa State University Press)

In other words, brick manufactures have a one component composition (the clay or shale). Now there is always an exception. Clays and shales happen to have some sulfate content. This is not good. You might get blistering or scumming or some other unpleasant defect.

What do they do?

They add barium carbonate or some other chemical to tie up the sulfates because barium forms insoluble sulfates. These materials are not premixed into the clay. They are just sprinkled on the clay and the extruder does sufficient mixing.

I’ll tell you a secret. Some other manufacturers may add barium sulfate such as the sanitary ware manufacturers that manufacture your bathroom fixtures. Some potters add it too.

Vanadium compounds cause scumming in otherwise pretty white bricks.

Defects caused by raw materials are usually related to impurities or particle size. The defect can turn up at any time in the process. Here are some examples of defects caused by impurities:

Impurity Defects

Lignite

Lignite 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.

Soda ash can react with soft lignite and turn it into a useful colloid that will keep the slurry dispersed.

In other words, some lignite can be beneficial in slip casting operations and other operations that use slurry. But the hard anthracite and other coal varieties are not good. They can cause blisters and pits during bisk and glost firing operations.

Clay companies have some control over the size and amount of lignite in their clays (often by blending.

You can run a screen analysis on the material and see how much lignite remains on the screens. If it is higher than the previous shipments, call the clay company and say these words: “What’s with all this lignite!”

That should get their attention. Once I said, “What is all this plastic?" (The filter bags had melted in the air floatation equipment. )

Make sure you blame them for all defects generated for the next six months. (Hey, you think I’m kidding! Make that the next two months or whenever that batch of clay is used up. )

Grit

We had a recurring blistering problem in fine china where I once worked and for years nobody could figure out what was causing it.

I was new with the company but not inexperienced in solving material problems.

I called each of our suppliers and said that we had blisters and it was their fault.

The representative from a kaolin company asked, “What’s the grit?”

I wasn’t sure what he was talking about because I had not used that particular clay before. Rather than tell him that I didn’t know what he was talking about, I said, “Hold on a minute. ”

I asked the quality control manager, “What’s the grit in that stuff?”

He had a questioning look on his face despite his 30 years of service. He pulled out the specification sheet for the last shipment, found that there was such a thing as “grit, " and gave me a number.

The number represented the amount of coarse material that showed up during a screen analysis of the clay. For this particular clay, the grit was in the form of mica.

Mica is not good!

Hearing the number, the representative said, “That is too high! I’ll call the mine!”

We set a control standard and never had that problem again.

I‘d paid my first years salary in five minutes a few days after joining the company.

Iron and Manganese Compounds, Silicon Carbide, Soy Beans and Salts

Sometimes clay manufacturers ship clays to storage areas by rail, ship, or barge. Dockside raw material storage is always dangerous for contamination. The reason is that these facilities ship iron ores, ferrosilicon, silicon carbide and they are not very careful about cleaning out a shed of ferrosilicon or other contaminant before loading it with a shipload of clay. (We changed from bulk to Super Sack® shipments. )

Some of these materials give off oxygen when they decompose. That occurs at some characteristic temperature.

We used the same British china clay in two different plants.

In one plant we had blisters in our decorated ware, the worst possible condition.

The other factory using the same clay didn’t. Why? The bisk temperature was much higher in the not affected plant. The contaminant, a manganese compound, decomposed before glazing and decoration.

The affected plant had both lower bisk and glost temperatures. The material did not fully decompose at these temperatures and bubbles were still appearing at the decorating temperatures.

The problem was complicated because we had to prove that the supplier caused a heavy loss that their insurance company should pay.

I’ll not go into the details because they proved we were right and did pay.

The reason for reimbursement was that I had requested them to never ship us material from a certain pit (mine). It had caused the company loses for years and I wanted no more of it.

They shipped from that pit rather than shipping us the processed clay that I specified.

Identification of impurities can be tricky without a Scanning Electron Microscope (SEM).

Heavy liquids can be used, but that is a nuisance. Microscopes are good for those who know how to use them. You can look at the contaminants from the heavy liquid separation.

Sometimes you must use an outside lab if you don’t have the correct equipment.

To isolate a contaminant before you send samples out, elutriate clays and screen non clays.

To elutriate a clay, take about ten pounds of clay and keep washing it down until only the contaminant is left. Put the contaminant in your body or glaze and reproduce the defect you are seeing in production.

Chloride can be detected from salts by washing the clay with deionized water and testing with a soluble silver nitrate solution. A milky precipitate is silver chloride. Salts cause blisters!

We had soy beans once in one of the plants I worked. The clay picked up the beans from a hopper car. They make big holes in your ware but they are easy to screen out if you have screening in your operation, which we didn’t!

There is a funny story that goes with the soy beans but I don’t have room to tell it here. I’ll save that for another article.

Particle Size

One of the most serious formulating errors is in not controlling particle size in the batch recipe.

For example, if you use too much of a certain clay having 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 must use some coarser grained clay(s) in your casting process. If you don't you will still be trying to cast a piece when the next millennium comes in.

If you want to have control of your casting process, then 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 maintain the total amount of kaolin and the total amount of ball clay to preserve color of the product.

Look at the particle size distribution of the clays you are using in your process. This information should be provided to you with each shipment.

Look at the particle size portion that says <0.5 microns. (If that information is not there, the <2 micron information usually is. Use it but it is not as good for control. )

The <0.5 microns is called the colloidal fraction. Keep the colloidal fraction the same in each casting batch. (Send me an email if you can’t figure out how to do this. Hint: if you have 30% Clay A in your batch and the colloidal fraction of the clay is 20%, the colloidal fraction added to the batch is 6%. )

You control your shrinkage by controlling particle size (and water content). Too fine a formulation will cause excessive shrinkage and you will get distortion and cracking problems.

John T. Jones, Ph. D. (tjbooks@hotmail.com, a retired VP of R&D for Lenox China, is author of detective & western novels, nonfiction (business, scientific, engineering, humor), poetry, etc. Former editor of Ceramic Industry Magazine, Jones is Executive Representative of International Wealth Success. He calls himself “Taylor Jones, the hack writer. "

More info: http://www.tjbooks.com

Business web site: http://www.bookfindhelp.com (IWS wealth-success books and kits and business newsletters / TopFlight flagpoles)

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