234 POTTERY. 



cast will necessarily burn white. A clay of this 

 kind must contain no calcareous or ferruginous 

 earth, and must also be carefully deprived of any 

 sand it may contain, by washing. It ought to 

 possess, besides, the capital property of shrinking 

 but little in the fire. If it should not prove suffi- 

 ciently ductile, it may be meliorated by the ad- 

 mixture of another sort. Last of all, it is beaten, 

 kneaded, ground, washed, and sifted, till it 

 acquires the requisite degree of fineness and 

 ductility. 



When, after this preparation, the clay has ob- 

 tained a due degree of ductility, it is rolled out in 

 small portions to the usual length of a pipe, per- 

 forated with a wire, and put together wdth the 

 wire, into a brass mould rubbed over with oil, to 

 give it its external form ; after wdiich it is fixed 

 into a vice, and the hollow part of the head formed 

 with a stopper. The pipes, thus brought into 

 form, are cleared of the redundant clay that adheres 

 to the seams, a rim or border is made round the 

 head, they are then miarked with an iron stamp 

 upon the heel, and the surfaces smoothed and 

 polished. When they are well dried, they are put 

 into boxes and baked in a furnace. 



In the Dutch manufactories, these boxes consist 

 of conical pots made of clay, with conical lids, with 

 a tube passing through the middle of them, by which 

 the pipes are supported ; or else, they are long clay 

 boxes, in which the pipes are laid horizontally, and 

 stratified with fragments of pipes pounded small. 

 Lastly, the pipes, when baked, are covered with 

 a glazing or varnish, and afterwards rubbed with a 

 cloth. This glazing consists of a quarter of a 

 pound of soap, two ounces of white wax, and one 

 ounce of gum arabic, or tragacanth, which are all 



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