Cement tiles are made of several layers of fine mortar comprising Portland cement, that is to say, a mixture of sand, marble powder, cement, and in the decorative layer, of coloured metal oxides.
After a chemical hydration reaction, the cement becomes very solid and hard, almost as hard as the initial rocks from which the constituents were extracted. These are the particulars that go into the making of cement tiles.
The mould used is made up of a sheet of steel polished flat, onto which is mounted a frame of heavy steel that opens and shuts thanks to a tightening device. The entire apparatus weighs nearly 15 kilos.
The bottom plate is first greased. Then onto it is poured, with a spoon or funnel, a mortar of fine consistency a few centimeters thick. The mixture consists of white cement and marble powder, coloured with ochres and metal oxides in powder form. It has a liquid consistency so as to avoid bubbles and other defects.
When decoration is wanted, first a divider is placed on the mould. This is a kind of stencil consisting of vertical partitions dividing the surface into compartments corresponding to the colour pattern. Formerly the device was made of foundry bronze. But due to high costs, tin-soldered brass dividers are used today.
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Hydraulic press from 1850 |
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The various colours are poured into their corresponding cells. The divider and the mould are finely adjusted to avoid smears from one cell into another. After all the colours have been poured in the divider is carefully removed.
Then this white cement mortar, which is lying backside up and will in the end constitute the surface of the tile, is dusted with a mixture of dry grey cement and sifted sand. This allows the hardening of the decorative layer.
Next the base layer is made with a grosser mortar consisting of ordinary cement and sand, mixed with a minimum of water.
Then a buffer – often stamping the brand name on the back of the tile – is inserted into the mould and the whole thing is put under press. The compression forces the excess water out of the decorative layer and into the base layer. After the pressure is released the tile must be removed from the mould. This is a delicate step, for the tile has not set yet and its removal demands great care. It is placed edgewise in the boxes, left there for several hours, then submerged in water. It is at this stage that the cement undergoes hydration. The boxes are
drained, then the tiles are stored and left to dry for at least a month before being put on the market.
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