|Monthly Tech-Tip |
It was first used in the late 1800s by Gladding McBean to make sewer pipe, subsequently for a wide range of architectural purposes. 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!
Although this material is known as a fireclay, it completely vitrifies by cone 10 (where it is near zero porosity). it's 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 bars begin 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 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 sticky as are 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 during shrinkage! This is very unusual.
Variation in the physical and fired properties of this material have 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 to dampen an variation encountered.
This unique combination of firing behavior 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).
The ZAM stoneware pottery clay body has been made for many years by various manufacturers on the west coast. It is:
15 Hawthorn fireclay
10 Lincoln fireclay
15 Ball clay
8 Grog or Sand
Another example is Soldner's Raku which is:
50 Lincoln fire clay
30 sand, all mesh
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.
The left two were made from a 100% mix of Lincoln 60, the right one adds 2% bentonite (Lincoln 60, by itself, matures into a stoneware at around cone 8). While it was possible to throw the pure material, the plasticity is a little lower than a normal pottery clay. That being said, it is smooth and has a soapy feel that makes it very pleasant to work with. A simple 2% addition of bentonite transformed it into a delight to throw! That only increased the drying shrinkage by about 0.5%.
Materials are not always what their name suggests. These are Lincoln Fireclay test bars fired from cone 6-11 oxidation and 10 reduction (top). The clay vitrifies progressively from cone 7 upward (3% porosity at cone 7 to 0.1% by cone 10 oxidation and reduction, bloating by cone 11). Is it a really fireclay? No.
|Materials||Lincoln 8 Clay|
|Materials||Imco 400 Fireclay|
ZAM clay body recipe
Fireclays are non-kaolin non-ball clay materials similar to stoneware clays but lacking fluxing oxides. Many fireclays have a PCE of 28 or more.
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
|Drying Shrinkage||5.5-6.0% @ 27% water|
|Firing Shrinkage||Cone 6: 7.5% Cone 7: 8.0% Cone 8: 8.5% Cone 10: 8.0%|
|Pyrometric Cone Equivalent||31 (claimed)|
|Sieve Analysis Dry||+35 mesh: 3.5 35-48: 3.0 48-65: 4.0 65-100: 3.5 100-150: 3.5 150-200: 4.0 200-325: 5.0|
|Water absorption||Cone 6: 3.0% Cone 7: 2.5 Cone 8: 1.5 Cone 10: 0.2|