Here at Coin Hunter we have 26 United Kingdom Royal Mint silver proof 50p coins created in 2019 to mark the life and work of Stephen Hawking. Every single coin has already toned on the obverse side, see images below.
Toning on recent coins in black foam, where the coins have been stored in the same conditions as older silver proof coins that show no signs of toning. The 2017 and 2018 coins are held in place with white foam.
The question we have to ask is: Does the black foam contain a chemical component that is not coin safe?
We also need to consider why only the obverse of the Stephen Hawking 50p is toning so quickly in the black foam. It should be noted that the screw capsule has the screw thread which will let in air on the obverse side - and that the reverse is sealed from the air by the tight fit of the foam itself.