Oilcloth was an 18th century shelter traditionally made of hemp or linen cloth with a linseed oil coating and was semi-waterproof.

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Oilcloth

“They (Canadians) name prelart a large and heavy cloth, oil-painted in red,… to keep oneself from the rain” Louis Franquet, 1752

"A course linen painted red with oil, with which we cover the [canoe cargo] as further protection against the rain." Louis Franquet, 1752

The fabric described during the mid 18th century was one aune wide (46 ¾ inches) and came in three different weights. The length of the issued cloth ranged from approximately 5 aunes or 6 aunes to 7 ½ aunes . This would mean that the cloth would be 233 inches to 350 inches in length or about 19 feet to 29 feet in total length. This information was derived from The Equipment of the New France Militia, Steve Delisle, pgs. 17 and 43.

It is often assumed that this amount of fabric would be sewn together to form a large tarpaulin in order to shelter the four. By cutting and sewing this amount of fabric you can make a rectangular piece of cloth that is approximately 7 ½ feet x 9 ½ feet if using 5 aunes of fabric, or 7 ½ feet x 14 ½ feet if using 7 ½ aunes.

Materials to make oilcloth
Hemp or Linen fabric - For a lightweight tarp a 12 oz material will work, for a heavier tarp the 14 oz.

Cotton- Many reenactors use cotton painters tarps in lieu of traditional materials. Once painted I suppose it is difficult to tell.

Linseed oil- Store bought linseed oil is still very acidic, even the boiled linseed oil. It can be used but in combination with the turpentine it will rot away the fabric over time. To neutralize the acid you can slowly heat the oil (outside and very carefully) and add calcium carbonate (limestone) to the oil. I do not have detailed instruction at this time on this process, but as I understand it you add the powdered limestone to the hot oil and it will smoke for half an hour and then be less acidic .

Turpentine - This is added to the oil after boiling in order to thin down the oil and make it easier to paint onto the cloth. Turpentine will also destroy fabric over time.

Red ochre- On Ebay you can search for iron oxide/red ochre/ferric oxide and buy 1 lb for under $10.00 and 5 lbs for under $15.00. This is the common color in the Great Lakes fur trade and during the French Regime in North America.

Instructions
Mix one part boiled linseed oil with one part turpentine.
Add red ochre till you get desired color.
Paint cloth and let it dry in the open. This can take several days or longer. DO NOT fold up wet fabric coated with linseed oil! It can spontaneously combust! Once dry it is fine.
Paint additional layers as necessary.

Another Option
An alternative to making your own paint would be to simply buy linseed oil based paint from a hardware store. Often called Barn Red oil paint, the ingredients are linseed oil, red ochre and sometimes a drier. This is as about as close as you come to a commercial alternative to making your own paint. Check the ingredients on the can, some barn paints have latex in them as well.