This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Following are instructions for a composition for cores that may be required for difficult jobs, where it would be extremely expensive to make a core-box for the same: Make a pattern (of any material that will stand moulding from) like the core required. Take a mould from the same in the sand, in the ordinary way, place strengthening wires from point to point, centrally; gate and close your flask. Then make a composition of 2 parts brickdust and 1 of plaster of Paris; mix with water, and cast. Take it out when set, dry it, and place it in your mould warm, so that there may be no cold air in it.