For practice the student may take either “ Stourbridge ” or “ Dinas ” fireclay and sand. The following table gives their average composition. The Dinas sand consists almost entirely of SiO2 (free), but in the Stourbridge clay the SiO2 is mostly combined with Al2O3. The clay is preferable for practice.
For reference regarding the composition of clays, the student may consult Tables of Analyses of Clays, etc., by Alfred Crossley. He will there find tabulated lists of analysis of all the most important fire and other clays, sands, etc.
The Chemical Analysis.—The analysis of fireclays much resembles that of silicates (previously given). For the present the student need only estimate SiO2 (free, combined, and hydrated), Al2O3, Fe2O3 and H2O. Then if, on summing up results, there remains unestimated more than 3%, he must proceed further with the analysis.
SiO2 (free, combined, and hydrated).—Estimate the total SiO2, by fusing 1 gm. clay (dried at 100° C.) with Na2CO3,K2CO3. Fuse for twenty to thirty minutes. Cool and treat as before directed (see Analysis of Silicates). The result is the total SiO2.
Free SiO2.—Take 1 gram clay dried at 100° C. and digest with H2SO4 (20E.) for about eight hours. Evaporate to dryness. Cool. Take up with water. Filter. Wash well. The residue consists of free + combined + hydrated silica + bases (if any). Transfer in small portions to a boiling solution of Na2CO3, (5E.), filtering now and then so as to replace the Na2CO3. When all the solution has been transferred to the platinum crucible containing the Na2CO3, continue the boiling, replacing the Na2CO3 till no reaction is obtained with NH4Cl (see Qualitative Analysis). This treatment dissolves out combined and hydrated SiO2 and leaves free SiO2. Dry, ignite, and weigh.
Hydrated SiO2.—Treat 5 gms. dried clay with boiling Na2CO3, omitting the H2SO4 digestion. Boil repeatedly. This dissolves out hydrated silica. Filter, add HCl and evaporate to dryness, and estimate the SiO2, which is reported as hydrated silica.
Combined SiO2.—Subtract free + hydrate SiO2 from the total 1 residue ’ free + combined + hydrated SiO2 + bases. The result is combined SiO2 + bases. If the ‘ total residue ’ is greater than the ‘ total SiO2 ’ previously found, the difference represents the bases along with the combined SiO2.
Water of Hydration.—Take 1 gram of clay dried at 100° C. and ignite at a red heat in a platinum crucible over the blowpipe till constant. The loss represents combined water. Al2O3, Fe2O3, Alkalies, etc. are determined as usual.
The student will find, on consulting works of reference, that a good fireclay should not contain more than 2% Fe2O3 and 1% of K2O or Na2O. That the importance of fixing such limits may be brought home to the student, the following ‘ fire-tests ’ should be performed :—
Tests concerning the fusibility of Fireclay.—Pass a sample of the dry clay through a 30 sieve. Mix one-third of the sample with a little water. Dry slowly. Place in a crucible in the wind furnace and burn at a good red heat. Cool. Crush and pass through the 30 sieve. Now mix one part of the burnt clay with two parts of the unburnt; add a little water, kneading well. Allow the kneaded mass to stand over-night. In the morning cut out with a sharp knife small test pieces like a T with the leg 1 cm. long and the cross piece about 2.5 cms. in length; width of face about .3 cm.; depth about 1.2 cm. Also cut small pyramids, square base, and abont 2.5 cms. high. Gently dry these pieces. Lute two E crucibles to a half brick (by means of clay). Set the half brick and crucibles on the fire bars. Insert the test pieces and lute on the lids, build up the fire. Fire at a red heat and raise the heat to a whiteness, continuing as closely as possible at this temperature for about two hours, using anthracite or the hardest coke for firing. Remove, cool, break crucibles, and examine the test pieces along the edges with a powerful lens. The edges should be sharp and unglazed. Take now three lots of the clay. To one lot add 5% Na2CO3, to the second add 5% Fe2O3, and to the third add 5% CaO. Prepare test pieces. Treat as before, and note the results.
Tests concerning resistance to Corrosion.—An absolute standard cannot be laid down, but a useful comparison may be obtained by testing the clay against some well known brand such as the “ Battersea.” With the mixture of burnt and unburnt clay, mould small scorifiers of approximately the same shape as the Battersea 5 cm. scorifier. Dry carefully, and burn at a good red heat. Place in the scorifiers the following charges: in one 20 gms. PbO, in another 20 gms. granulated copper and 1 gm. borax. Place similar charges in two Battersea scorifiers. Transfer the four charges to a muffle between a red and white heat. (For safety, insert in the muffle a flat tile to catch any metal that may run through.) Continue the fusion till one scorifier is perforated. Remove, pour, and examine for corrosion. Note the results. If time be available, the student may proceed to study the effect of varying proportions of different impurities on corrosion.