1831.] 
On Water Cements. 
69 
§ 5 . — Of Artificial Water Cements. 
22. A mixture of carbonate of lime with pure silica, and a mixture of carbonate 
of lime with pure alumina, will sometimes, but very rarely, combine into a water 
cement. 
23. A mixture of 5 measures of carbonate of lime, with 2 measures of any mix- 
ture of silica and alumina, will generally form a water cement, but one that 
hardens very slowly, and which is not always quite perfect, as you may prove, 
by mixing 5 measures of chalk, with 2 measures of pipe clay. 
24. To the above mixture of carbonate of lime, silica, and alumina, add either 
the protoxide of iron or the carbonate of magnesia, in a proportion which need not 
exceed one-fifteenth part of the whole compound, and this will form an artificial 
water cement equal to those of nature. A much less proportion of the protoxide 
of iron will answer the purpose. The most convenient for use is that which is 
obtained, by collecting the scales struck otf from red hot iron, by anchor-smiths, 
at work in a dock-yard. 
25. The above mixtures of chalk and pipe clay, with the protoxide of iron, or the 
carbonate of magnesia, being rather expensive, you may use, in preference, a mix- 
ture of 5 measures of chalk, with 2 measures of the common blue or brown clays 
of nature, as the latter contain iron in combination, which pipe clay does not. 
26. Every compound used as an artificial cement, must have its ingredients 
reduced to a state of impalpable powder, and intimately mixed with a moderate 
quantity of water : then moulded nearly to the size of common bricks, but using 
water instead of sand, to prevent adhesion, and afterwards cut into cubes, of about 
2^ inches each side ; which must be compressed by beating, moderately dried, and 
burned in a common lime kiln, to the degree before specified in Article 3; after 
which, they must be ground to an impalpable powder, and secured in air-tight 
casks, like the natural cements. 
27. A proportion of fine coal dust, of about one-twentieth part of the compound 
of chalk and clay, should be mixed with the raw cement, previously to the mould- 
ing and burning. Any oil, tar, or other combustible, of which coal tar, next to 
coal dust, is the cheapest, will answer the same purpose. The object is to restore 
the iron in the clay, as much as possible, to the state of protoxide, during the 
burning of the mixture. 
28. Coal dust is not absolutely necessary to all artificial cement mixtures, but 
it does no harm to any, and our experiments have proved that it often does good. 
It is not altogether an additional expense, for it helps to burn the artificial cement, 
with which it is incorporated, with less extra fuel than would otherwise be re- 
quired. 
29. Five measures of chalk, and 2 measures of the blue clay of the Medway, 
make a good cement, if mixed without excess of water, and burned within a fort- 
night after the clay is taken out of the river. A long exposure to air gradually 
changes the colour of this clay from a very dark blue, almost to a dull or dirty 
white, and robs it also of its chemical properties, so as to render it altogether unfit 
for a water cement, when mixed with chalk alone.* This is probably owing to the 
gradual decomposition of animal and vegetable matter, contained in the clay. The 
like unfavourable effect is produced in a shorter time, by a great excess of water, 
or by repeated washings, when it is about to be mixed with the chalk. 
30. The brown clays, being of a much purer quality, do not appear to be liable 
to such deterioration by exposure to air ; but they have the disadvantage of being 
generally coarse, and, therefore, incapable of forming an efficient cement when 
mixed with chalk in their natural state. They require, either to have the coarser 
