Significance of "haymarks" on British coins

So how do you explain copper spots and copper streaks that are so common on US gold coins? Since copper has a lower melting point than gold, it should be completely absorbed into the gold, right?

 

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That would seem to be my best explanation as well, though its remarkable how many of them end up graded by NGC and PCGS even though evidently cleaned.

 

Perhaps someone can explain how this coin "toned" the way it did:

 

If the gold melt is allowed to cool too long or too slowly, as the metal solidifies the different metals will try to separate from solution based on density and melting point. This can result in the outer surfaces of the cast ingot being slightly richer in copper than the interior. When the ingot is rolled out this results in areas or spots of higher copper content on the surface of the strip.

 

Doesn't look like tone or streaking to me. I would grade it "original uncleaned covered with black crud AU50" --Incidentally, what did NGC grade it as?

 

The coin graded NGC-55.
http://www.teletrade.com/coins/lot.asp?auction=3088&lot=1212

The "black crud" is in all likelihood due to copper discoloration (and is not foreign matter). Here is an explanation about ugly spotting on gold coins. While it is focusing on gold coins, it would also apply to silver coins.
http://www.taxfreegold.co.uk/redspotsongoldcoins.html

 

I am familiar with red spotting, I have a Czech ducat from 1931 that is spotted so bad its tempting to quarintine the coin lest some of the others catch the disease. But seriously..,,The spots, are red to dark orange, nothing at all like the surface of your half-eagle. The spotting on your coin looks like it is held on by adhesion of some sort, not as a discoloration of the metal itself. Not unlike this recent coin from Bowers & Marina.
http://www.stacksbowers.com/auctions/AuctionLot.aspx?LotID=230832

 

firstly tin has only ever been used in halfpence and farthings both of which contained a copper plug,
secondly the production of tin in the uk was almost exclusively from cornwall, wales produced copper and birmingham never had mines of any description.