To tan a fur that is cased you need to make a tanning solution that the hide can be submerged into. Fox are pretty easy to do because they are usually pretty small. You could do it in a five gallon bucket.
Here is a tanning solution from another
post that will work for fox as well.
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Here is a method we use for doing 6 wild rabbits.
(Please remember that you have to be very gentle with them. They will fall apart very easily.)
Required for this method:
2 - TK03 Tanning Crystals Double Pack
1 - TK02 Tanning Oil Bottle
2lbs - Non-Iodized Salt (We recommend Hay & Stock Salt) usually $4.00 for 50 lbs.
1 - 5 Gallon Bucket
Method for 6 Rabbits:
Get a 5 Gallon Bucket
Put in 2 gallons of water, 1.5 lbs of Tanning Crystals, 2 lbs of non-iodized salt
stir it thoroughly and then add the rabbits.
Let the rabbits sit in the solution for 3 days stirring occasionally.
Pull the rabbits out and let them drain for 20-30 min.
now lay them out flat and apply a very light coat of our tanning oil to the flesh side
let them air dry.
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You may want to check out this
post for making the fur fluffy.