| Description |
Grog is a generic term referring to granular material made by grinding brick or other fired ceramic (although it can also be made by crushing certain natural minerals or artificially by calcining mineral and grinding it). Grogs are added to bodies to improve drying performance, reduce drying shrinkage, improve fired abrasion resistance, reduce thermal expansion, reduce fired shrinkage, reduce density, impart visual character, etc. Refractory grogs are available as well as ones with low thermal expansion.
Grog is marketed in designations that refer to the size range of the particles. 20-40 mesh grog means that all material under 40 and over 20 has been graded out to provide a material whose particle size range is between these sizes. A -40 grog (minus 40 mesh) is one that has particles that range all the way from 40 mesh down to fine power sizes. Companies are not always successful in maintaining their materials within the range they specify. In addition, two materials from different companies with the same designation, 20-40 for example, can be quite different since one may have the majority of sizes at the coarse end of the range and the other at the finer end. Companies manufacturing grogs face technical challenges to maximize the amount of usable material (since the grinding process produces alot of particles that are too fine to be used). Coarser grogs are thus more difficult to manufacture. Thus, as you might expect, grogs can be expensive.
Simplistically, grog additions improve drying properties simply because the clay shrinks less and because the individual grog particles terminate micro cracks before they become big cracks. But the porosity of a grog's particles, their surface texture and their shapes are also important in the dynamics of how it affects clay body working and drying properties. The more grog that is added to a body more its plasticity will be impacted (finer grogs have a larger impact). While it might seem that adding a grog with a wide range of sizes to a clay body will produce the maximum benefit, this is not normally the case. Generally, the highest percentage of the largest particles that can be tolerated should be employed, this has the greatest reducing effect on the shrinkage and the least reducing effect on the plasticity. Amazingly, plastic modelling and pottery throwing bodies can still feel quite smooth when they have 20-40 mesh grog particles as long as the body is very plastic. Often silica sand is incorporated with a grog addition to supply another specific range of particle sizes.
It is possible to achieve a pressing or non-plastic forming body with almost zero shrinkage by mixing a body with 90% or more grog. The heavy refractories industry commonly uses a 50% 4-16 mesh, 10% 20-36 mesh, 40% 40-60 mesh mix of grog and a deflocculated slip to bind them together for drying. This principle can be extended to almost any grogged clay body.
Grog is also used to increase pore space in the matrix to enable quicker venting of water vapor during drying and gases of decomposition during firing. A sculpture clay body, for example, typically has 15-25% grog (but can have much more).
Since grog is typically prefired, its does not normally undergo a firing shrinkage (unless the body in which it is a part is fired to a temperature higher than the grog was initially fired at). This means that the more that is put into a recipe, the less the clay shrinks during firing. The effect can be quite dramatic. For example, a clay body that normally has a fired shrinkage of 5.5%, but with an addition of 15% 20-48 grog and 15% 70 mesh silica sand the fired shrinkage drops to 2.5% while maintaining the same degree of vitrification. Fired properties of grog-containing bodies are also affected by the hardness, color, porosity and impurity content of the grog particles. |