548 NEW YOKK STATE MUSEUM 



this. Sucli substances go Tinder tlie collective name of grogs, 

 and may include sand, ground bricks, coke, graphite, etc. 



Grogs serve to prevent cracking in both burning and drying. 

 They also tend to prevent the blistering of easily fusible, ferrugin- 

 ous clays when fired hard. They furthermore add to the 

 porosity of the ware and thus facilitate the escape of the moisture 

 in drying and in the^ early stages of burning, and also enable the 

 product to withstand sudden changes of temperature. Grogs may 

 however act as fluxes at high temperatures; the finer the grog, the 

 more intense will be this action. 



If the grog is to decrease the shrinkage in drying and burning, 

 it must not be added in the form of powder, but as grains, and- 

 :even in this case, the grains must not exceed a certain size, other- 

 wise they will only serve to increase^ the tearing of the wares in 

 drying and burning. The cause of this lies in the fact that the 

 grog itself does not as a rule shrink, and if in any one place the 

 clay substance shrinks to such an extent that it can no longer sur- 

 round the particle of grog, the latter will act as a wedge, tearing 

 the grains apart, and a crack will be started. If this action shows 

 itself in the raw material, it can be eliminated by the addition 

 of powdered grog. When this is not possible, the coarse particles 

 must either be removed or reduced by crushing. 



Sand. This is the form of grog commonly found in nature and 

 most frequently used artificially. Sand as it occurs in nature is 

 commonly cO'mposed of mineral grains, representing a variety of 

 species. Pure quartz is of course the most desirable, but quartz sands 

 generally contain impurities, which at times may be suflicient to pre^ 

 vent their use for certain purposes. Clay impurities might be 

 washed out, but, as others like feldspai", calcite, etc., could not be 

 removed by washing, the best way tO' obtain clean quartz sand is 

 to crush up vein quartz, or quartzite. An advantage connected 

 with this type of sand is that the grains have an angular structure, 

 whereas grains of natural sand, being usually of sedimentary origin, 

 have a rounded form, and will not interlock as well. In addition 



