I had a similar problem a while ago and I got really tired of it.
The sparex I was using did not only contain small light brownish
granules, but also thicker one (maybe two which stuck together).
They look more brownish. When heating the pickle, they create a
filthy sort of slime. I suspect that not only does this slime not
pickle the piece, it actually makes it dirty.
That brown gunk seems to be an impurity found almost universally in
any package of Sparex pickle. My guess is that they’re buying their
sodium bisulphate as an industrial byproduct, rather than as a proper
technical grade chemical. Perhaps it’s uses previously in some
process where some oil or corrosion preventative ends up
contaminating the chemical. Don’t know. But the junk certainly does
no good for the metal or the pickle that anyone I know has ever been
able to discern. I once tried to actually phone up Grobet (the
distributor) to inquire about the gunk. A sales rep on the other end
who must have thought I was an idiot, tried to tell me that the brown
stuff was my assurance that I was buying genuine Sparex pickle, as
though this were some badge of honor or something. I told her I
suspected she didn’t actually know what she was talking about, and
that ended the conversation.
Since then, instead of buying anything labeled as jewelers pickle
(they’re all a bit overpriced) I buy the sodium bisulphate packaged
for pool, hot tum, spa use as a means to lower the ph of the water.
This product can be found in hardware stores. labels like “Spa
down”, “Ph-down”, and others. It’s essentially pure sodium
bisulphate, the same chemical in sparex, with the addition of a few
percent anti caking agents, so the powder pours and doesn’t clump up.
It’s inert in the pickle. With this stuff, that costs about half of
what sparex does, or less, no more brown junk making a mess of
everything. Works great as a nice clean pickle. If you need more than
small amounts, any decent chemical supply company can sell you
technical grade sodium bisulphate, and this sort of source might be
the cheapest of all.