Waterproof fabric basically comes in two types, totally (or nearly totally) waterproof fabric which typically has a plastic or rubbery texture, and fabric which is merely water shedding. The former type is often made by coating the fabric in a drying polymer such as linseed oil, or some sort of rubber extract. Linseed oil is an excellent product for this use as once it dries it is waterproof and not damaged by solvents. Fabric coated this way is used to create oilskin garments, such as the ubiquitous stockman’s jacket popular in Australia.
You would be hard pressed to find a better book on the many uses of linseed oil, than Stephen Shepherd’s “Shellac, Linseed Oil & Paint“. It is, admittedly, a little varied in writing quality, but it’s packed with useful information on linseed oil and various additives that can make it behave in different ways.
Shepherd also has an excellent book on hide glue (gelatin) which is also occasionally used in waterproofing textiles, often in the form of a very watery solution called “size”. Sizing, or applying size to the warp, is also a technique used occasionally by weavers working with linen to protect the fibres and reduce friction. Hide glue (and size) can be removed later using warm water, which would tend to make it not terribly useful for waterproofing cloth, except that it can be made waterproof with the addition of Alum or other additives. See Shepherd’s hide glue book for more details.
In the second category of fabric treatments are those treatments which merely make the fabric shed water to some degree, without making it totally impermeable to air or water. Here in New Zealand, the best known example of this is the Swanndri, an iconic NZ garment which is a baggy jacket made of tightly woven wool and treated so that it sheds water rather than absorbs it. This garment has been a popular choice for NZ farmers for about a century, although it is sadly now made in china.
The Swanndri waterproofing recipe is a closely guarded secret. The original maker of the garment, William Broom, apparently dipped the finished garments into the waterproofing solution which caused the garment to shrink unpredictably, hence the baggy fit.
While we don’t know what recipe Mr Broom used, we do know what technology for making water resistant cloth was around at the time, and as a starting point, this article indicates that Mr Broom actually had a copy of a particular book on his shelf: “A Fortune in Formulas“, first published in 1907.
This book quotes many different recipes for making cloth water resistant, but most seem to revolve around the applicate of metal acetates or stearates, with the most common metal being Aluminium. For example, the following recipe:
An easy method is the formation of aluminum stearate in the fiber of the cloth, which may readily be done by immersing it in a solution of aluminum sulphate in water (1 in 10) and without allowing it to dry passing through a solution of soap made from soda and tallow or similar fat, in hot water. Reaction between the aluminum sulphate and the soap produces aluminum stearate and sodium sulphate. The former is insoluble and remains in the fiber; the latter is removed by subsequently rinsing the fabric in water.
An alternative recipe in the book is very similar, but starts with Aluminium Acetate. The chemical reactions here seem fairly straightforward, and I can understand them with just a high school knowledge of chemistry. The chemicals themselves are “fairly safe” to experiment with if investigated thoroughly in advance. Material Data Sheets are available on the internet for each chemical used, and you should ensure a thorough understanding of each chemical reaction and it’s products before embarking on experimentation in this field.
In general though, the idea of causing a chemical reaction to occur on the cloth itself, resulting in the depositing of aluminium stearate on the fibres (not coating the yarn, but the individual fibres within the yarn) sounds similar to waterproofing techniques still used today. Aluminium stearate is still used as a waterproofing agent, and is not generally regarded as being toxic, although I’d be interested to know about how to avoid pollution if doing this on an industrial scale.
Once you know the word to search for, you can google waterproofing recipes with “Alumium Acetate” and “Aluminium Stearate” and you’ll find lots of interesting results, including expired patents. I’m a bit unclear on whether the book “A Fortune in Formulas” is itself out of copyright, so I wont post a link to it here, but it is possible to find digital copies of it elsewhere on the internet.