This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
Plumbers' soil or smudge is made in small quantities, as it deteriorates if kept. To make a soiipotful, place in the pot 1/4 lb. of size or diluted molten glue and a little water; gently warm until the size dissolves, but do not boil. Mis 1/2 cub. in. of chalk ground to a fine powder with a pennyworth of lampblack, and then with a pallet knife incorporate some of the melted size with the mixture on a flat board or stone to form a thin paste, after which place the whole in the pot, warm, and stir together thoroughly. Try the soil on a piece of lead; if when dry it peels off, add water; if it is rubbed off easily, the size is not good, or the lead is greasy. Old and thick soil is thinned with porter or stout, but do not add too much or the soil will become so sticky that the solder will cling to it.
 
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