• Materials for Ceramics

Lincoln 60 Fireclay

Description: Dark buff-firing plastic stoneware

Notes

It was first used in the late 1800s by Gladding McBean to make sewer pipe, subsequently for a wide range of architectural purposes. To this day they continue to provide it as a raw powdered material from Lincoln, CA.

It is also used as a major ingredient in many commercial West Coast USA stoneware and middle and high fire pottery clay and sculpture bodies. This material is very smooth and has a unique feel that many potters can recognize with their eyes closed!

Lincoln Fireclay is confused with this, it is a different material.

This material completely vitrifies by cone 10 (where it is near zero porosity). Its porosity decreases steadily from cone 6 to 10, but it is beginning to bloat at cone 11. However, the fired shrinkage increases until cone 8 after which it begins to expand (indicating over-firing). It would be an acceptable stoneware, by itself, at cone 7-8. With a 5% addition of feldspar it should be possible to make Lincoln fireclay work as a cone 6 stoneware. If crazing occurs, blend in a high-silica cone 6 porcelain recipe (or a silica-feldspar mix with a little bentonite to maintain plasticity).

Lincoln Clay has several other very unusual properties also:
-It has excellent drying properties (resistance to cracking) even though it has high plasticity. It's drying shrinkage is below 6%.
-It is very plastic, like a ball clay, yet it feels like a kaolin (it is not as sticky as other clays of the same plasticity).
-27% water is required to make the Lincoln clay plastic enough to work for pottery (whereas a typical plastic pottery clay body is 20-22%). Yet it still has a fairly low drying shrinkage! This is very unusual.

Variation in the physical and fired properties of this material has been a concern with its use in pottery. Its sieve analysis also varies considerably by batch. For this reason, it is common to mix it with other materials.

This unique combination of firing behaviour and workability made this an ideal material for making vitrified unglazed sewer pipe. Those same properties make it possible to create a pottery body having a very high percentage of this material (90% is feasible).

It is also common to find recipes where Lincoln is blended with feldspar, silica and ball clay (where the ratios approximate or equal 50 ball clay, 25 silica, 25 kaolin). This is just a standard whiteware, so in effect, Lincoln is being blended with a porcelaneous stoneware.

Imco 400 Fireclay is similar to this material.

Related Information

The ultimate testing instrument to measure plasticity:

A potter's wheel. With an experienced potter.

This picture has its own page with more detail, click here to see it.

The left two leather-hard mugs were made from a 100% Lincoln 60 Fireclay (from Gladding McBean). By itself, the clay matures into a stoneware at around cone 8. While the pure material has a pleasant, smooth, soapy feel and can be thrown on the wheel, the plasticity is lower than that of typical pottery clay. The mug on the right adds 2% bentonite. That simple addition transforms it into a delight to throw! And only increases the drying shrinkage by about 0.5%.

Numbers on data sheets simply do not convey the difference the bentonite makes. But an experienced potter can feel it immediately. That makes a potters wheel (and throwing experience) a valuable laboratory testing instrument for a comparative assessment like this. There is no absolute measure for plasticity, so we most often simply say that one body is more or less plastic than another.

A fireclay that is not really a fireclay!

This picture has its own page with more detail, click here to see it.

This is a Lincoln 60 fireclay drying disk (that has been fired to cone 10R). It has near zero-porosity and is dense and very strong. It is like a stoneware clay, quite vitreous.

Is Lincoln 60 really a fireclay? Simple physical testing says...

This picture has its own page with more detail, click here to see it.

Materials are not always what their name suggests. These are Lincoln #60 Fireclay test bars fired at cone 10 reduction (top) and from cone 11 down to 6 oxidation (top to bottom). This clay already has stoneware density at cone 7 (3% porosity as indicated by our SHAB test). It vitrifies progressively from there upward (less than 3% porosity at cone 7 to near zero% by cone 9 oxidation. Maximum firing shrinkage happens at cone 8 and by cone 10 it is expanding (indicating decomposition has started) and it is bloating by cone 11 (melting is sealing the escape of gases of decomposition). Is Lincoln #60 a really fireclay? Absolutely not! But at cone 6 it is a credible plastic stoneware, all by itself!

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