He'll be your opponent. The salt will interact with the ice, causing it to melt more quickly. The ice will leave its impression on the salt—you will be able to see in which positions pieces sat for the longest after the game is over and the players have left. And the warm fingers, grabbing the ice, will play against time. It should be a game with similar but different, slightly skewed conditions than the ones chess players are used to. (Especially when it comes to calculating timing.)
The weekly chess games are part of Cornish's group show Game Theory, which welcomes indeterminacy, mess, and melting, in the name of past Cornish accompanist John Cage, and his desire not to say anything about chess fanatic Marcel Duchamp (who did not abandon art for chess, as is legend: he secretly created this unbelievable thing instead).