In part 1 we looked at a number of formulas used in patternmaking as well various recipes for moulding sand. This second (and concluding) section deals with core sands and chemical treatments for finished castings.
Core Sands
Apparently our pommie friends used to use – and may still use – a “compo” mixture of ordinary moulding sand and ground fragments of graphite crucibles in making their cores. American (and Australian) practice favours the use of oil-bonded cores, with a fairly coarse high-silica sand as the base. Sea sand, minus the shell grit, is ideal.
40 parts sea sand (by weight) to one part linseed oil is suitable for most ordinary work. Part 5 of the “Backyard Foundrywork” series describes how to go about actually making a core.
Other materials used as binders to strengthen core sand include clay, flour, resin and horse manure. A few pre World War II British recipes:
- For large cores: 5 parts new red sand, 5 parts used moulding sand (“floor sand”) and 2 parts horse manure or – I kid you not – 1 part cow hair.
- For ordinary cores: 10 parts new red sand, 2 parts horse manure.
- For small cores: 6 parts floor sand, 6 parts sea sand, 4 parts red sand, moistened with linseed oil, or 4 parts sea sand, 6 parts red sand and 1 part flour, moistened with linseed oil.
Treatment of Castings
Commercially the process of “dressing” castings – smoothing rough edges, knocking off scale and sand grains etc. – takes up a considerable part of the foundry’s operations. Rough and ready iron castings – railway brake blocks come to mind – are usually “rumbled” in a large, horizontal rotated drum together with a few shovels full of small pieces of scrap cast iron which act as an abrasive. Smaller and/or more intricate iron castings are generally shot or sand-blasted. Metal “fins” and scale on larger iron castings can be chipped away with hammer and cold chisel, and heavier imperfections ground off. Similarly non-ferrous castings are ground, filed and buffed to whatever finish is appropriate for the particular job.
Pickles
Acid “pickles” are sometimes used to clean iron castings. A common one is sulphuric acid pickle: I part commercial concentrated sulphuric acid to 30 parts water. The castings are left in the pickle for about 10 hours, then washed with hot water to remove all traces of acid and prevent rusting.(ALWAYS add the acid to the water (slowly), never vice-versa. The head generated when you pour water into concentrated sulphuric acid causes local boiling, which can spray acid and acid vapour everywhere, including over you). Wear gloves and safety glasses (A face shield is even better) or goggles. (I don’t need to say why, do I?).
Dips
No, Fred, not the sort that you hand around at the barby.
Pickled iron is dull and black (as opposed to pickled people, who are just plan dull). Polishing, of course, restoes the original colour. However a “bright dip” can do it much more quickly.
A bright dip for iron is made from hydrofluoric acid – 24oz, nitric acid – 10oz, zinc metal – 2oz, and water – 2 gallons. Tie the casting securely with heavy copper wire, dip and swirl around in the solution for a few seconds, rinse in cold followed by very hot water and dry in sawdust. If you leave the castings in the dip for more than a few seonds they may be damaged, and the solution will be deactivated.
A bright dip for copper brass or bronze consists of 4 parts hydrofluoric acid, 3 parts nitric acid and a small handful of table salt. Handle castings as above.
A satin-finish dip for brass can be made from 1 part hydrofluoric acid to 3 parts water. Other satin finishing dips are as follows –
Hydrofluoric acid 2 parts, nitric acid 1 part, muriatic (hydrochloric) acid ½ part. Water 5 parts.
Or, hydrofluoric acid 2 parts, nitric acid 1 part, water 10 parts.
