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Hi ,

. Clean fresh water should always be the first method before resorting to using soap. Steve is right, but its the tension across a sheet of the canvas that gives its ability to create surface tension across any water that hits it and keep whats under the canvas dry . Old square rigged sailing ships used canvas stretched drum tight over timber frames as hatch covers for the holds using this principle (sailing is my other hobby in case you wondered). But as its very hard to get canvas drum tight in a tent, and tent canvas is'nt the same weight as sailing canvas, a waterproofing compound is applied to either the thread the canvas is woven from or the woven canvas itself. Tencate canvas that Pennies are made from uses treated thread.  Sunlight degrades the treatment so after a few years  the makers proofing will have deteriorated anyway. So getting a helping hand in the cleaning of old canvas by using pure soap will create no great loss. But it could be with new canvas.

Its the rinse that's the real trick - you've got to get every last trace of the soap out so the reproofer can stick,  which you can with pure soap. Other modern detergents have long named chemicals that hang around after the rinse which is what stops reproofing from being effective.  

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