|Monthly Tech-Tip |
In ceramics, clays with added aggregate are free formed or molding into shapes that are slowly dried and fired in a kiln. The process requires much skill.
More info coming soon.
This was made at the Medalta Artists in Residence program in 2014. It celebrates Jim Marshall, a local historian and ceramic artist who does architectural murals.
This is a sculpture body named Industrial Crank from Potclays in the UK. I dried some out, slurried and screened the grog out then dewatered the remaining clay to get this. There is almost 50% grog. Yet this body is known for amazing plasticity and toughness. How is that possible with this much grog? That base clay. It is extremely sticky and plastic. Yet it has only 6% drying shrinkage. The grog has a narrow range of sizes, from about 35 mesh to 70 mesh (a high percentage is of the coarsest size). Yet amazingly, the body does not feel coarse. Why? Again, it is this clay base. Each grog particle is nestled in a buffer that firmly holds it yet gives it freedom to move. I am working on a complete report and will share from my Insight-live.com account soon.
This body is a plastic fireclay base having 13% 20/48 grog and 10% 65 mesh silica sand. But the texture is far coarser than one would expect. That is because it has 4 cubic feet of perlite per thousand pound batch. If desired the surface can be trowel smooth. This works well partly because the perlite particles are soft and easy to crush.
It is easy to find pictures of spalling bricks at google. This happens because water trapped in the pores of ceramic and concrete expands during freezing and breaks it down. This should be a concern to people making architectural and sculpture pieces for outdoors. How do you know if your ceramic will spall? It needs open and closed pore space if over 1% closed porosity. Measure the percentage gain in weight (of a test tile) after a 24-hour water soak, that is 'open porosity'. Then boil for 5 hours and it will soak up more water, measure that as the % closed porosity. The second number needs to be 20% greater than the first. Why? The closed space provides a place for expansion as the water is freezing.
These bi-body strips are made by rolling two clays together in a thin sandwich. Three porcelains are being compared to a very plastic grogged sculpture body. After drying (top) they curl a little, two toward the sculpture body and one, the most plastic of the porcelains, toward the white. But on firing to cone 8 they curl dramatically toward the porcelain side (because it shrinks alot more). Now imagine one of these porcelains is being used as a engobe on this body.
Here is the oversize (from Plainsman MSculp) on the four coarsest screens we use to do particle size distributions on clay bodies. There are very few intermediate sizes between the very fine particles of the base body and the coarse particles of the grog and sand. Contrary to what I have thought up until now, lately we have found that this approach makes for greater plasticity, better drying and less water splitting than if the grog and body contain a range of coarse to very fine particles. It also feels smooth on the wheel.
A cone 10R sculpture clay containing 40% ball clay, 10% kaolin, 10% low fire redart (for color and maturity), some quartz and 25% 20x48 grog. This fine grained base produces a body that feels smoother than it really is and is very plastic. It is even throwable on the wheel.
Examples of various sized grogs from CE Minerals, Christy Minerals, Plainsman Clays. Grogs are added to clays, especially those used for sculpture, hand building and industrial products like brick and pipe (to improve drying properties). The grog reduces the drying shrinkage and individual particles terminate micro-cracks as they develop (larger grogs are more effective at the latter, smaller at the former). Grogs having a narrower range of particle sizes (vs. ones with a wide range of sizes) are often the most effective additions. Grogs having a thermal expansion close to that of the fired body, a low porosity, lighter color and minimal iron contamination are the most sought after (and the most expensive).
This is an example of how soluble salts can enhance the appearance of the fired surface of a cone 10R clay. This sculpture body is a vitreous dark brown burning base having lighter colored 20 mesh grog particles. The one on the left uses native stoneware clays that contain natural flux-containing solubles that migrate to the surface during drying. When fired they act like an extremely thin layer of glaze, producing a darker sheen on the surface. The thickness (and thus color) varies with contour and exposure of the surface during drying. The inside of the cone has no solubles at all.
This terra cotta sculpture, called “HOPE" was made at the studio of Stephanie Osser, a collaboration with my former student Jamaal Eversley. With Covid, Jamaal had to finish it with acrylic paint. HOPE was exhibited during the 2 months of the Boston Exhibit called “ Real Friends” summer 2020.
Grog is a term used in ceramics to describe crushed brick (or other fired ceramic) aggregate that is added to sculpture and structural clays to improve drying properties.