Grog,also known asfiresandandchamotte,is a raw material usually made from crushed and groundpotsherds,reintroduced into crude clay totemperit before makingceramicware. It has a high percentage ofsilicaandalumina.
It is normally available as a powder or chippings, and is an important ingredient inCoade stone.
Production
editIt can be produced by firing selectedfire claysto high temperatures before grinding and screening to specific particle sizes. An alternate method of production usespitchers.Theparticle size distributionis generally coarser in size than the other raw materials used to prepare clay bodies. It tends to be porous and have low density.
Properties
editGrog is composed of 40% minimumalumina,30% minimumsilica,4% maximumiron(III) oxide,up to 2%calcium oxideandmagnesium oxidecombined.[1]
Its melting point is approximately 1,780 °C (3,240 °F). Its water absorption is maximum 7%. Itsthermal expansion coefficientis 5.2 mm/m[clarification needed]andthermal conductivityis 0.8 W/(m·K) at 100 °C and 1.0 W/(m·K) at 1000 °C. It is not easilywettedbysteel.[1]
Applications
editGrog is used inpotteryandsculptureto add a gritty, rustic texture called "tooth"; it reduces shrinkage and aids even drying. This prevents defects such as cracking, crows feet, patterning, andlamination.The coarse particles open the green clay body to allow gases to escape. Grog adds structural strength to hand-built and thrown pottery during shaping, although it can diminish fired strength.
The finer the particles, the closer the clay bond, and the denser and stronger the fired product. "The strength in the dry state increases with grog down as fine as that passing the 100-meshsieve, but decreases with material passing the 200-mesh sieve. "[2]About 20% grog is added to crude clay (in the dry form) before mi xing with water. Adding grog to clay serves two primary functions: 1) It helps prevent cracking of the clay when the ceramic piece is being worked and when it dries, by reducing its plasticity; 2) it protects the ceramic piece fromthermal shockwhile firing, particularly, at the sudden rise or lowering of temperature, and which, if not added, can cause breakage. Substitutes for grog used in pottery are dried and sifted horse manure, or sand collected from dry riverbeds (which has been sifted through a screen), or finely groundschist.[3]Others make use of volcanic ash. Some natural clays already contain an admixture of some "natural temper," for which cause the potters who make use of the clay do not add any temper of their own.[4]
InMiddleandSouth Europe,grog is used to create fire-resistant chamotte type bricks and mortar for construction of fireplaces, old-style and industrial furnaces, and as component of high temperature application sealants and adhesives.
A typical example of domestic use is apizza stonemade from chamotte. Because the stone can absorb heat, you can bake pizza or bread on the stone in a regular domestic oven. The advantage is supposed to be a more even heat. A normal commercial domestic oven cools down when the door is opened. The stone however remains hot, creating a more even bake. Another advantage is the fact that the stone can absorb some moisture making for a drier bake.
Archaeology
editInarchaeology,"grog" is crushed fired pottery of any type that is added as atemperto unfired clay. Several pottery types from the EuropeanBronze Agearetypologizedon the basis of their grog inclusions. The practice of adding grog to clay as a temper was widespread throughout many cultures, and is mentioned in the writings ofHai Gaon(939–1038), who wrote in his commentary on theMishnah,compiled in 189 CE: "ḥarsit[= grog], that which they grind [of potsherds] and make therewith clay is called [in Hebrew]ḥarsit."[5]
See also
editReferences
edit- ^abJain 2003,p. 82.
- ^Liddell 1922,p. 503.
- ^Fontana, Bernard L.; Robinson, William J.; Cormack, Charles W.; Leavitt, Earnest E. (1962).Papago Indian Pottery.Seattle, Washington: University of Washington Press, on behalf of the American Ethnological Society. p. 57.OCLC869680.
- ^Andy Ward's Ancient Pottery,How Mata Ortiz Pottery Is MadeonYouTube,Feb 2023, minutes 0:44–2:46; 4:18–4:52.
- ^Hai Gaon(1921), "Hai Gaon's Commentary on Seder Taharot", in Epstein, J.N. (ed.),The Geonic Commentary on Seder Taharot - Attributed to Rabbi Hai Gaon(in Hebrew), vol. 1, Berlin: Itzkowski, p.19(Keilim10:2),OCLC13977130(Available online, at HebrewBooks.org:The Geonic Commentary on Seder Taharot- vol. 1
- Jain, P. L. (2003),Principles of Foundry Technology(4th, revised ed.), Tata McGraw-Hill,ISBN978-0-07-044760-8.
- Liddell, Donald Macy (1922),Handbook of chemical engineering, Volume 1(unknown, revised ed.), McGraw-Hill Book Company,ASINB000UFRYQI.