This is the paste of the English flint-wares; its composition varies within certain limits, but usually it contains -

From 20 to 45 per cent, of white plastic clay, which serves as a binding substance; From 25 to 45 per cent, of kaolin, to prevent the paste from turning yellow in firing; From 20 to 40 per cent, of flint, which shortens and gives whiteness and durability; From 5 to 15 per cent, of pegmatite or felspar, which acts as a flux.

The clays are always washed to separate the coarse parts; the flint is heated in kilns and then crushed, and the felspar is also pulverised. The purified substances are diluted in a volume of water so measured that they shall have the same degree of consistency, and then a calculated quantity of each paste is poured into a vat, in which the mixture is effected by stirring. After this, the paste is filtered through fine brass wire sieves which are shaken by machinery; the filtered portion is sent to the filter press and thence to the cellar to solidify.

Moulding, in the case of small output, is done by hand; for larger quantities machines working on firm or dry paste are used. In this case the paste, when it comes from the filter press, is carried to hot-air drying-rooms, and is then blended and reduced to powder in a centrifugal pulveriser. The two operations are performed by the same machine (Fig. 821).

Fig. 821. Centrifugal Pulveriser.

The powder is afterwards placed in a metallic mould and compressed in a press similar to those described on P. 433.

The quarry may be decorated in various ways. In the case of plain glazed quarries, a boracic plumbiferous alkaline glaze is used; some formulae are appended (Pressel).

Composition of the Frit.

Composition of the Glaze.

Boric acid ....

..

17

..

18.5

15

..

18

Borate of soda (borax)

35

30

..

..

32

..

Carbonate of soda (crystals)

..

22

23.5

16

..

20

23 of fe1spar.

Carbonate of lime (chalk) .

18

17

15

13

..

16

..

..

..

..

..

11.5

..

Carbonate of lead (white lead)

18

21

12

18

6

18.5

22

Kaolin ....

12

15

11

8

11

3

13

Plumbate of lead (minium).

..

..

..

..

19

..

Pegmatite ....

..

..

31

25

..

41

..

30

31

8

22

..

11

18

Sand.....

35

28

17

10

26

24

32

..

..

6

..

..

..

3

Frit.....

..

..

..

..

..

..

..

52

47

74

60

70

59

60

The glaze is applied by one of the processes mentioned in the general remarks on glazing (immersion, insufflation).

When the quarries are to receive a pattern in relief, the outlines are impressed on the clay when still fresh by stamping in plaster moulds; in ordinary patterns the outline is marked out in black lines, and in both cases the spaces between the lines are fitted in with enamels, generally transparent. This is what is wrongly called cloisonne decoration.

When a large number of quarries are decorated with the same monochrome pattern, copper-plate printing is used, and for polychrome decoration chromolithographic printing. This decoration is performed over glaze, and sometimes under glaze.

Besides, we may use, according to the effects required, one or other of the processes mentioned in the general remarks on the decoration of pottery. The quarries of which decorative panels, intended for the facing of walls, are formed are most frequently covered with transparent enamels.