THE ART OF THE TILE - MASON
Second of a series of articles on the designing, manufacturing and laying of clay tiles by "Ceramico".
In our last article we saw how a 8" X 6" white glazed tile from the clay state was made and how it received its initial firing so that it became what is known to the trade as a biscuit tile, the word biscuit being the best way to describe the rough porous surface of the tile prior to the addition of the glaze.
Break a glazed tile, look closely at the jagged edges of the break and you will see, resting on the surface of the tile body, a very thin line of a glass-like substance. This is the glaze and the next process, after having fired our tile into the biscuit state, is to apply a coat of glaze, and fire again, this time through “glost".
The method thus described of examining the glaze, whilst serving our purpose in this particular instance, is not the best way to do so since it is the continual aim of every tile manu- facturer to fuse his tile body and his glaze into one indis- tinguishable whole. Indeed in a first-class white glazed tile a magnifying glass would be required to pick out the glaze across a break as described above.
Bearing in mind that the object of this series of articles is to describe the art of the tile-mason and not to dwell at long, expert length upon the manufacture of the tiles that he uses, we can briefly say, at the risk of being accused of ambiguity, that glazes fall into two categories-Lead glazes and Leadless glazes.
A lead glaze, as the name implies, is a glaze composition in which the basic material is lead in the form of a carbonate or oxide of lead.
Lead is the best flux known to the business and possesses many excellent qualities including transparency, stability, and an ability to fuse at fairly low temperatures. This latter quality means that "fugitive" colours, which would be lost at high temperatures, can be held in perfect suspension at com- paratively low heats and thus their brilliancy of colourings produced.
A drawback of lead, of course, is that it is a very dangerous commodity to work with and would seriously endanger the health of the operatives were it not for the special precautions taken which nowadays reduce the risk of infection to a negligible factor.
Leadless glazes are considerably safer and therefore easier to handle but by leaving out the lead compounds the manu- facturer not only places a severe limit on his range of colours but also sacrifices his brilliancy of finish.
An "Earthenware Glaze" is a leadless one and it is this type of glaze that is applied to the white earthenware biscuit tile.
Since the glaze and the body of the tile, after their union, have to be put through a fierce heat and then cooled off it will be obvious that their two co-efficients of thermal expan- sion and contraction must be, as near as possible, the same. If this were not so the one material would strive against the other during the two processes with the result that the glaze would either "craze" or "shiver". The word craze means that the tile surface becomes covered with fine minute cracks after the fashion of a cobweb, and shivering describes the flaking away of the glaze from the edges of the tile.
In the preparation of lead glazes some manufacturers use the various lead compounds in the raw state and without any previous preparation mix them with the other ingredients such as china clay and flint. Metal mineral oxides are added for colouring. However, by far the most widely used method is the one known as "fritting" and this is more satisfactory in many ways.
To look at, frit is like powdered glass and this is what it literally is. In preparation, the ingredients are mixed and heated in crucibles until they melt together into a molten whole, the heat range being 1,200 to 1,400 degrees centigrade. The molten mass is run off into a tank of cold water, and the sudden meeting of the two extremes of temperature causes the frit to break up into small particles of glass, ready for the grinding mill, which grinds the particles into a very fine powder. After grinding it is placed into labelled containers ready for future use.
The ingredients for the glaze, sometimes all embodied in the frit, and in other cases only a proportion, the others to be added afterwards, are now all mixed with water and ground in a mill. This further grinding is carried out to a very fine degree indeed and the glaze slip is then sieved through silk and passed over electric magnets for the purpose of removing any particles of iron which might have been carried through the various stages of processing.
A glaze, when in use, must possess a constant density and the china clay content helps towards this end by keeping the heavier ingredients in a state of constant suspension.
Coloured enamel tiles are coated with glaze by the dipping of the face of the biscuit tile into a basin full of glaze. This is done by hand and the even coating of the surface requires a great amount of skill.
The modern method of glazing white tiles is to pass them, on a moving belt, over a roller that is saturated with the glaze. This method is used solely for the glazing of white because the extensive use of white requires the manu- facturing and accumulating of large stocks. By keeping the machine exclusively to the use of white no cleaning is required and, in any case, the roller method is unsuitable for anything but plain straightforward colours.
A further method is to blow a over the tile by compressed air.
fine spray of glaze
Mottles are obtained by first of all dipping the tile into one glaze, dipping a sponge into another glaze then dabbing the glaze-laden sponge onto the surface of the already once- glazed tile.
The glaze after being applied to the surface of the tile becomes powdery as it dries, which it does very quickly. Sub- sequent handling before firing must be done with the utmost caution.
In their preparation for the furnace kiln the tiles are "cranked" which means that they are placed face-upwards on a sort of triangular fireclay shelf. Tiles do not touch each other because if they did they would be fused together when passing through the intense heat and would come out of the kiln stuck together and useless.
Practically all the glost firing of tiles is done through tunnel ovens which were introduced by Dressler about 1910. Many people to-day, refer to all tunnel ovens as Dresslers although various types of ovens by other manufacturers and patentees are now being widely used.
Tunnel ovens can be of almost any length with a practical minimum of say 150 to 160 feet. The heating medium may be either producer gas, town gas, or electricity.
The centre section of about 30 feet is heated to 1030-1040 degrees centigrade, which heat is constantly maintained.
The cranked tiles are placed upon trucks which are pro- pelled continuously through the tunnel at the rate of about sixty inches an hour. Thus they slowly reach the furnace section and just as slowly leave it. This, of course, is ideal as a gradual heat-up and cool-off is required to preserve shape etc. the same as for biscuit.
As the final stage in the process, the tiles, after firing are again sorted with great care and discriminately graded as to tone and quality,
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