In the manufacture of all ceramic products the requisite quantities of components (flint, stone, ball clay, china clay, bone, etc.), are measured in terms of weight or volume then subjected to a mixing process.
For most ceramic bodies, the actual mixing takes place in slips form, which ensures an intimate blending of the constituents and hence a uniform product. The original quantity of each component may, however, be measured out in the dry state or in slip form.
The component powder materials are simply weighed out then transferred to the mixing ark-due allowance being made for any moisture content
An older technique, sometimes used in the manufacture of bone
china, is to fill and level-off a “standard box” with the dry powder component. A specified number of boxes of each material is then used in the body mix. This of course, is an attempt to measure the dry materials on a volume basis. It suffers from errors due to inconsistent packing, and variable moisture contents of the raw materials.
The most widely used method, in British practice, is to obtain a stock of each constituent material in slip form. If the consistency (pint weight) of each slip is known, then a given volume may be pumped into the mixing ark-and it is a simple matter to calculate the dry material involved.
E.g. A mixing consists of 960 pt of fireclay slip (at 25 oz per pt ) and 120 pt of flint slip (at 30 oz per pt). What is the percentage dry recipe of the mix [assume that both components have a specific gavity (dry) of 2·50]?
Using Brongniart’s Formula:
1 pt of Fireclay slip has a dry content of (5X5) :3 = 8 1/3 oz
1 pt of Flint slip has a dry content of 10× (5:3) = 16 2/3 oz
Total amount of dry Fireclay = 960× (25:3) = 8,000 oz
Total amount of dry Flint = 120× (50:3) = 2,000 oz
i.e. the dry recipe is 80% Fireclay; 20°/o Flint
The volume of each component slip may be measured in any
convenient units: pints, gallons, litres, cubic feet, etc. In particular,
the simplest method is to use a “dip-stick” in the mixing ark to
measure the depth of each component slip as it is added to the mix.
Provided that the ark has a uniform cross-sectional area, the depths,
in inches (often referred to as “wet inches”), correspond to volumes.
Hence a “wet” recipe takes the form:
14 in. ball clay at 24 oz per pt
8½ in. china clay at 26 oz per pt
5 in. flint at 30 oz per pt
2¼ in. stone at 32 oz per pt
lf other units of volume are substituted for “wet inches” the
composition of the final mix is unaltered. There will, of course, be
a change in the total quantity of body slip in the mix. Nevertheless,
the recipe may be written as:
14 pt ball clay at 24 oz per pt
8½ pt china clay at 26 oz per pt
5 pt flint at 30 oz per pt
2¼ pt stone at 32 oz per pt
and it now becomes a simple matter to convert this into the corresponding “dry” recipe. Assuming that the specific gravity of each of the component materials is 2·50, the dry recipe becomes:
Ball clay 14×(24-20) x (5:3) = 93·3
China clay 8½×(26-20) x (5:3) = 85·0
Flint 5×(30-20) x (5:3) = 83·3
Stone 2 ¼ ×(32-20)x (5:3) = 45·0
and the percentage dry recipe is:
Ball clay 30·4%
China clay 27·7%
A further simplification is possible if the specific gravity of all the dry components is the same. In the above example, the specific gravity of each component was 2·50, and the factor 5/3 appeared in the expression of each dry content. This constant factor (5/3) may be omitted, since it does not affect the ratios between the weights of the component materials.
Dry recipe E.g.
Ball clay 14×(24-20) = 56
China clay 8½×(26-20) = 51
Flint 5×(30-20) = 50
Stone 2 1/4×(32-20) = 27
This is virtually the same dry recipe, which is clearly seen when it is converted to percentage form, giving:
Ball clay 30·4%
China clay 27·7%
In general, then, if the specific gravity of each component is the same, to convert from “wet” recipe to a “dry” recipe, apply the formula “Wet inches”×(P-20) to each component.
E.g. Calculate the percentage dry recipe of the following mix:
10½ in. ball clay at 24 oz per pt
6½ in. china clay at 26 oz per pt
4 in. flint at 32 oz per pt
2½ in. stone at 32 oz per pt
Ball clay = 10] × 4 = 42
China clay = 6½× 6 = 39
Flint = 4 × 12 = 48
Stone = 2½×12 = 30
Percentage dry recipe is:
Ball clay 26·4%
China clay 24·5%