1920 Halfcrown, Weak Strike, Bunc But What's The Problem?

Hi! I am new to this forum, but the answer to the odd coloring on your half crown is to do with the silver alloy from which the coin was made. in 1920, the bullion value of silver skyrocketed, so The Royal Mint in Tower Hill in London reduced the purity of silver in the silver coinage from .925 fine to .500 fine. The alloy they used for a couple of years was 50% silver, 12 1/2% nikel, and 37 1/2% copper.

The coins after about a year or so developed an ugly greenish yellow tinge. In 1922 the Mint felt compelled to alter the alloy mix again to solve this problem.

The answer they come up with was more successful; with an alloy they called 'Quaternary Metal' in which the alloy was 50% silver, 40% copper 5% nikel, and 5% zinc. The planchets before striking were blanched, that is, washed in weak acid, to leach some of the copper and zinc from the surface of the planchet, so when it was struck, the coin had a surface enrichment of silver, thus solving the problem of discoloration

I'm new to half crowns also. Technical question; is the strike weak due to the relief depth on the cote of arms? I would have thought that area of the cote of arms would have been a low point. Is it because the strike does not allow the metal to flow to the high points and therefore looks weak and worn? (Kind of like the eagles breast on a Morgan.) It is fascinating, I only have half crowns with Elisabeth on the Obverse and the Rose Seal reverse.

The striking of all GB coins in the period around the end of WWI was very poor.
Several theories have been proposed, but the main ones are varying striking pressures or the height of the obverse bust causing metal displacement, thus weakening or blurring certain areas (this is also the reason for ghosting on EDVII and GV bronze).
Needless to say a well struck example commands a premium.
John.

anyone has a idea? Maybe svslav or sap?

I suspect sel_69l has the situation correctly figured out. The coin does not, to me, bear the hallmarks of PVC residue, which generally manifests itself in specific areas rather than an overall haze. With that said, there is no harm whatsoever in an acetone bath for it, as long as you do your research here and practice safe handling procedure. An initial soak of an hour or so, followed by a rinse in fresh and an additional soak of a few hours' duration, will conclusively prove the issue - after such a procedure, the green will be gone if it was due to PVC residue. Actually, the initial soak should make the difference, but I'm obsessive that way.

I caution you again, research safe handling first. Acetone is vigorously flammable, moreso than gasoline, and must be treated with respect.

Superdave, Argumentandcoins and svslav, thank you all for helping me out solving that coin. Even though being a weak strike, it's still a very lovely coin, it's a straight uncirculated with almost no bagmarks. I put it into a Lighthouse 2x2 yesterday and it went to my per type folder, replacing a VF 1921 Halfcrown of same type. In the hand, the light green toning is actually pleasant to the eye, even though I am not a big fan of toning. I think Sel_69l also deserves a credit here, he's figured out what's going on with the coin. Again, many thanks for making me feel relieved. The seller offered me a full refund, yet, I'll write him an email today saying that the coni is fine.