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LXXXI. On the Porcelain Clay and Buhr-stone of Halkin 
Mountain, Flintshire. By W. Bisuor and Co., of Nant y 
Moch, near Holywell in that County*. 
Tar qualities which fit a stone for grinding corn, especially 
wheat, are hardness, to prevent it as much as possible from 
wearing down by the constant friction to which it is exposed, a 
certain degree of tenacity, to prevent the grinding surface from 
scaling or chipping off, and a cellular structure, in order to in- 
crease the quantity of cutting surface, the walls of the cells being 
at the same time thick enough to resist the strain upon them. 
The advantages hence resulting are, that the flour is in no 
material degree contaminated by the mixture of earthy particles 
abraded from the stones, the grinding is expeditiously performed, 
the bran is completely disengaged from the flour, and the flour 
itself is very little heated by passing through the mill. This 
latter circumstance is of great importance, it being found, by 
experience, that flour over-heated, or killed, as the technical 
phrase is, will never produce bread so light as that which is 
greund cool. 
In some parts of the valley of the Seine and of the adjoining 
districts in which the fresh-water limestone occurs, is found a 
siliceous rock, in detached masses or blocks, of various size, 
known on the spot and in commerce by the name of buAr, It 
is a substance intermediate between hornstone and calcedony, 
and possesses, in an eminent degree, the qualities which pecu- 
liarly fit it for grinding wheat. All the fine flour required for 
the supply of the metropolis and of the other large towns in this 
island is prepared by means of millstones of French buhr, a cir- 
cumstance which, beside rendering us dependent on foreigners 
for so essential an article, is the occasion in time of war of enor- 
mously enhancing the price, and subjecting our millers to great 
inconvenience. 
The northern shore of the Isle of Wight is the only district 
in this country in which the fresh-water limestone has hitherta 
been found, but it does not appear to contain any buhr-stone. 
The entrochital chert or hornstone (vulgarly called screwstone) 
which occurs interstratified with the mountain limestone in Der- 
byshire, as it resembles bubr-stone in quality and texture, has 
occasionally been made trial of for a grinding-stone, but always 
unsuccessfully on account of its fragility and softness.— Sec. 
In the year 1816 Mr. Thomas Hooson, of Flint, observed on 
* From the Transactions of the Society for the Encouragement of Arts, 
Manufactures, and Commerce, for 1821. The Society's Isis gold medal 
was voted for this communication, 
Halkin 
oe 
