How to clean a Fieldstone fireplace that has darkened over the years?

Mix one ounce of soap and one ounce of table salt with enough water to make a cream. Rub mixture into brick surface with cloth; allow to dry at least ten minutes and remove with stiff brush. (the stiff brush part sounds like hard work-salt is not known for it’s cleaning ability)

We just painted ours, I was so tired of the dirty gray looking stone. At least now it looks clean.

I would try a paste of baking soda and water. Scrub it with a brush, then wipe with clear water to rinse. If all else fails, paint it or white-wash it.

Try some muratic acid. If really bad, you can grind down the outer surface using a stiff wire brush attachment on a drill. Then i would seal it.

I have sprayed mine down with vinegar then put a layer of baking soda on top let it sit then used a push broom to scrub it and wash off baking soda or let it dry and vacuum with brush attachment.