Grime On Gold Sovereigns

An obvious first step is to give the coin a long soak in acetone, then rinse with distilled water.

If the coin is in a slab, the slab must necessarily be broken to clean the coin.

Although I completed a type set of 17 Australian sovereigns and half sovereigns, which included included an Adelaide Pound Type 11, 9 Sovereigns and 7 Half Sovereigns, none of these were slabbed. Slabbing was entirely unsuited to my method of display and storage of the completed type set.
All of the coins in my Australian type set were washed using soap and water with the skin on my fingers softened and cleaned first, so that I could rub the coins safely. 22 ct gold is much harder than the skin of water/soap softened fingers.

Do not use this method of cleaning for cameo mirror proof gold coins. These sorts of coins normally come encapsulated from the mint, so should never need cleaning anyway.
However,
even gold coins in worn fine condition really gleam, when gently washed with good old soap 'n water.

If the stain still remains on the coin pictured, one should really question if the alloy is correct to 22ct gold specification.
One approach to prevent cleaning is to slab the coin.
I personally wonder why, if it was slabbed in the first place, that the stains were not removed beforehand.

The proper term that would be used is
" original skin " in that the coins referrred to above had never been
Dipped or cleaned .

RE dirt quite common on Shield Sovs due to the tin silver zinc copper mix
sovs at that time were often weighted on brass scales thus rubbing on high points and brass left on coins leading over time toning stain on high points.

gold is always mixed with zinc copper tin and silver leading different colours of gold, pre 1900 sovs have slightly different colour than modern ones

As I understand it 22ct gold sovs were alloyed with refined copper.
Thus, any trace element that is not gold or copper would be at an extremely low level.

22ct gold coins with copper alloy are nevertheless very capable of toning over a period of many decades, if left undisturbed. There are many examples of this phenomenon that can be found on uncirculated examples, especially of British Guinea coins in the British Museum.

There are two exceptions to this that I am aware of. Both relate to issues from the Sydney Mint.
Arsenic was found in some sovereigns that were tested by The Royal Mint in London, because some of them were found to be brittle, and the presence of a trace amount of arsenic was found to cause the brittleness.

All Sydney Mint sovereigns and half sovereigns from 1855 until 1866 were alloyed with silver, and not copper. These coins have a richer, but slightly paler golden color, that is seen in all grades.
The Royal Mint in London objected to this alloy, and so it was changed to standard copper alloy after 1866.