I've been collecting ancient and medieval coins for several years.
Occasionally I buy bulk uncleaned lots and work with them.
Your best bet for cleaning any dug ancient coins is this :
-- soak in olive oil (depends on encrustation level, but I've
had them soak for as long as 2 weeks; the longer the better)
-- remove from olive oil and briefly dip in soap/water
-- gently wipe clean with soft towel
-- repeat soaking if heavily encrusted
-- if encrustation is particularly bad, you can *gently* use a
*soft* brass brush
In general, coins that are ground dug will have collectable value
based on rarity, not condition. In general, anything dug will be of
relatively low value and low condition anyway and anything that you
do to them in the cleaning process will be negligible to what water,
minerals, and fertilizer in fields did them over 2000 years. The
highest quality coins are not typically dug straight from the
ground, they were buried in something that protected them from the
elements (vase, jar, metal armpit coinpurse that was used by Roman
Roman coinage was devalued several times between AD 100 and AD 450.
In the mid-3rd century you have a proliferation of the silver washed
antoninianus. In dug coins, most of the silvering will already be
removed, but you should take care, just in case.
All that being said, I've been able to get XF quality coins from dug
lots, but they are rare. 85% are G-VG (i.e. junk), 10 % might be
F-VF (typical bronzes ranging in value from $10 - $35 depending on
rarity and type), 5% or less might be VF+. It is not impossible to
find silver and gold, but 99.99% of these are cherry picked before
they're offered for sale.