I'm trying to figure out the right ratios to scour cotton.

I'm doing it in the washing machine, weighing the fabric before I put it in the machine and adding two kettles full of boiling water, and letting the load sit with the lid up for about an hour. 

I've seen scouring instructions on a natural dye blog that says 1/2 pound gets 1 t (teaspoon/5ml) synthropol and 4t soda ash, and instructions on Pro Chemical and Dye that say 1/2 t synthropol and 1/2 t soda ash per pound. Which means one says to use 4 times the synthropol and 16 times the soda ash as the other. Both say an hour and make it really hot. Other dye blogs say that using synthropol at this point isn't as important, and that any good regular detergent will do. 

So how much soda ash should I be using, since that's the biggest variation in recommendations. 
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Jacquardmod Jacquardmod
One thing I have found for dyeing is that recs are all over the place.  Different people have had success with different methods, and the will use what has worked for them. 

So, how can you tell if your scour works?  Dopes getting good results tell you that?  Well, some people don't scour at all, and get pretty good results.  You can always try both and see what works, but I think the synthrapol and soda ash suggested on that natural dye blog are pretty high.  

You are really trying to get any grease(soda ash stripping this) and breaking surface tension(with the synthrapol) in the scour.  Everything is about opening the fibers up and letting any junk out.  I would use the Prochem instructions until it doesn't work.  Soda ash is stronger than people think it is.  

Keep in mind that natural dyes have almost no affinity for the fiber like synthetic dyes do, so results there are much more likely to be thrown off by contaminants in the fiber.  
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