Our resident history nerd has filled us in on the particulars of canvas through the ages - if this is your thing, take a read:
Cotton fibres swell when wet, and stay somewhat swollen, which closes the gaps between the threads, making woven cotton fabric waterproof. In the old days, one would need to throughly wet any canvas items before using, to activate the waterproof properties.
The traditional 'olive green' colour of canvas came from the method used to 'rot proof' the cotton fabric, which involved treating with a copper sulfide solution - the copper imparting the green colour. This was an effective method of manufacturing a reasonably strong, waterproof fabric which would withstand the elements, using a combination of the natural swelling of cotton fibres, plus usually a wax based treatment to ensure the waterproofness.
If manufactured items were sewn with a cotton thread, this gave very good protection as the thread would also swell to fill the holes made when sewn.
Despite these useful natural characteristics of the cotton fibres, tearing and loss of strength proved to be a big problem, so canvas manufacturers tried various methods to increase the strength of the fabric. The most effective method was proved to be by using a blend of polyester and cotton fibres to manufacture the threads the canvas is woven from.
Today, Black Duck uses a unique blend of cotton and polyester to achieve maximum durability, water resistance and comfort!