Mortar . 
.272 
stitute for ivory black, which is prepared in the same way 
from ivory* The coarser powder of these, is what I un- 
derstand by powder of charred bones. But when this is 
not the manufacturer’s design, the door of the iron still 
is opened whilst it is hot, and the charred bones which 
flame and burn when they meet the air, are thrown into 
a kind of kiln, at the bottom of which the air can freely 
enter, and maintain the combustion, until the bones are 
burned to whiteness, for the greater part. The white 
fragments are picked, and rather bruised, than ground, 
to a gritty powder, by a millstone which rolls on them 
vertically over an inclined circular plane. This powder 
passed through a sieve is called bone-ashes, which are 
much used in metallurgy, and fitter for our purposes in 
incrustations, than the powder of burned bones ground 
as pigments are. The fragments which have not been 
thoroughly burned in the kiln, form a dark grey powder ; 
and mixtures of the white and grey burned bones afford 
bone- ashes of the lighter grey colours. 
The whole quantity of bone- ashes, which is to be used 
in the same incrustation, ought to be well mixed ; for it 
is impossible to sort the well burned or the grey bones so 
accurately as to secure an unity of colour in the parcels of 
powder which are successively prepared, and a very small 
variation of colour will be seen in the incrustation.” 
Water Cement , is made on the continent by mixing 
with goodmortar, the ochry earth Puzzuolana, or a simi- 
lar earth called Traas or Terras in England, where it is 
imported from Holland. It is mixed accurately with 
the mortar in proportion of about one third or one fourth 
of the lime. It is obvious that if precautions are neces- 
sary in any case to secure good and perfect lime, they 
will be necessary in the case of water cement. 
