A common surface defect in fired whiteware and porcelain ceramics. Fired specks mar an otherwise clean surface.
This is normally occurs where materials are contaminated with natural iron minerals (e.g. iron stone concretions) or by iron from the refining process. Porcelain manufacturers must be especially careful to avoid this problem. One tiny black speck can ruin an entire large fired porcelain item.
Although most industrial minerals state on their data sheets the particle size specifications they adhere to, almost all whiteware and porcelain ceramic product manufacturers test the materials they use for contamination that might contribute to specking in fired ware. Typically, any iron-containing particle remaining on a 150 mesh screen is a threat to most white burning bodies, glazes or engobes. But most must go further. Dry clay powder cannot be physically sieved beyond about 50 mesh (the screens blind), the only way is air separation or slurrying and screening. Thus many product manufacturers slurry, sieve, filter press and then pug their bodies; this tremendous effort is the only way to assure a speck-free fired surface. Manufacturers of clay bodies for potters are unable to adhere to this same level of care (for cost reasons). All they can do is monitor incoming materials and employ magnetic separation somewhere in the powder flow (users of their porcelains must learn to tolerate some fine specking).
If you are using a gas kiln and have experienced increasing levels of specking in fired ware, consider disassembling and cleaning your burners (they may have iron or oxide dust buildup inside).
The materials and body were clean. The problem was very strange because the specks only appeared on the insides of the ware. The problem turned out to be iron powder in the burners (shown in the overlay in this photo). Disassembling and cleaning them solved most of the issue. The rest? Disassembling and cleaning them better!
This is the plus 100 mesh particulate from 50 grams of two different ball clays. Most of the particles are carbon, they will burn out and possibly cause glaze defects. If any of them are metallic, they will produce fired specks.
Notice the inside of this large transparent glazed cone 6 stoneware bowl. There is a concentration of specks on one part because that area was sponged at the leather hard and dry stages to smooth surface problems that happened during the jiggering process. These specks are normally driven below the surface during forming.
These are jiggered lids made from Plainsman M340 middle temperature stoneware. The one on the right was sponged in the dry stage to smooth issues that occurred during jiggering. That has exposed speck producing particles that were under the surface. This body is made from quarried materials that are ground to 42 mesh.
Using stonewares it is easy to get pretty sloppy in the studio because a particle of iron or cobalt in a glaze or body is no big deal. But on a ice white, translucent, transparent-glazed piece it is a really big deal. These specks are particles of cobalt that were trapped in my 80 mesh glaze screen from previous use. I use a soft brush to coax the glaze through the screen faster, but even that was enough to dislodge some of the cobalt particles. The lesson: I need a dedicated glaze screen for use with this transparent glaze, it gets used for nothing else.
The left half of this cone 6 buff burning native-clay stoneware (Plainsman M340) was sponged at the dry stage. That exposed iron-bearing particles that are normally pushed under the surface. The result is a denser population of fired specks. While not usually a problem on flat surfaces, this can be an issue when rims of functional pieces are sponged and glazes stretch thin there during firing.
Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.
These are the oversize particles (from the 70, 100, 140 and 200 mesh sieves) from 100 grams of a commercial ball clay. They have been fired to cone 10 reduction. As you can see, this material is a potential cause of specking, especially in porcelain bodies. It is not only wise to check for oversize particles in clays, but firing these particles will tell you if they contain iron. A 200 mesh screen would be a good start for this test, it would catch all of these.
The stated particle size of a material and fired appearance can both be misleading. For example, these are Volclay 325 bentonite particles fired to cone 8 oxidation. They are from a water washed sieve analysis test, the oversize particles from a 325 mesh screen (left) make up 2% of the total and 1% are from the 200 mesh screen (right). Although the 325 particles appear ominously dark, individually they are likely to small to produce apparent fired specks in a porcelain. However 200 mesh sizes can produce visible fired specks, but that fraction of oversize does not have nearly as high iron or flux content. Still, the finer darker particles could agglomerate, it might be better to use a cleaner bentonite to plasticize a porcelain.
These are two strong bar magnets that are suspended below the chamber of a hammermilll that grinds stoneware clays. This iron they hold is both natural in the clays and from wearing of the hammers during grinding.
Five different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The glazes have been sieved to 100 mesh but remaining specks are still due to agglomeration of particles, not particle size differences.
5 different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The specks are not due to particle size, but differences in agglomeration of particles. Glazes employing these iron oxides obviously need to be sieved to break down the clumps.
These are very hard, high in iron and can be as large as volkswagens. Tiny iron concretion particles cause specking in fired ware, especially in reduction.
This cone 6 white opacified glaze has an addition pigment-bearing granular mineral to create speckle (e.g. illmenite, manganese granular, ironstone concretions). This speckling mechanism can be transplanted into almost any glaze. Unfortunately, the metallic particles that produce the speck are often heavy and settle quickly in the glaze slurry. This can be prevented somewhat by flocculating the slurry.
The reduction was fairly heavy and this piece went to cone 11. The tiny iron stone concretion particles melted vigorously and flowed. This is why clean firing results requires 200 mesh materials!
A sought-after visual effect that occurs in reduction fired stoneware. Particles of iron pyrite that occur naturally in the clay melt and blossom up through the glaze