Fire clay comes from the same source that sand does. It is almost pure oxide of alumina, which is separated out from the sand by a combination of the chemical action of the waters of the streams. Fire clay has traces of the other impurities mentioned in the analysis of molding sands. It is found in the lowest strata of the deposit beds. It is used to mix with fire sand in the proportion of 1 to 4 as the daubing mixture for cupola and ladles.
Clay wash is a mixture of fire clay and water. The test for mixing it is to dip the finger into the wash and then withdraw it, whereupon there should be an even film of clay deposited on the finger. Clay wash is used as the basis of heavy blackings. It is used as follows: for wetting crossbars of flasks; for breaks in sand where a repair is to be made; to wet up the dry edges of ladle linings when repairing with fresh daubing mixture; in fact, any place where a strong bond is required at some particular spot.
Parting sands or parting dusts must contain no bond. They are used to throw on to the damp surfaces of molds which must separate one from another. They prevent these surfaces formed of high bond sands from sticking to each other.
The cheapest parting sand, and by far the most commonly used, is obtained by putting some burnt core sand, from the cleaning shed, through a fine sieve.
Beach sand is also used as a parting sand, but the rounded nature of its grain weakens the molding sands more than does burnt core sand.
Charcoal facing dusted from a bag makes an excellent parting dust on fine work.
A dust manufactured expressly for the purpose and called "Par-tainol" is the most perfect material for fine work. This is applied from a dust bag. It is not only useful for sand joints, but is a great help if there is a deep lift on a pattern where the sand is liable to stick, or for a troublesome box in the core room.
Although the materials for this purpose - flour, rosin, oil, etc. - are on the purchasing list of the general foundry buyer, for the purposes of this article they will be explained in detail under Core Work.