How can I wash the soot off of my horses?

I would get a good wash mitt and either Orvus shampoo or Dawn dish soap (its gentle enough to clean the oil off ducks after oil spills) and scrub, scrub, scrub. You might have to wash a few times and if you are really worried about drying them out you can also get some spray conditioner such as Eqyus Avocado Mist or Healthy Hair Care Hair Conditioner (both are awesome leave ins) but I wouldn't be too worried about it since this is not a daily occurance just a special situation.