Re: at what temperature does salt water freeze?

Dear Matt,
Yes, at atmospheric pressure, pure water freezes (and melts) at 32 deg. 
Fahrenheit, which is zero degrees Centigrade. The temperature, at which 
ice and water coexist, is lowered by increasing pressure. Skating relies 
on this fact.
Two different things are actually involved in your experiment:
A tense string exerts local pressure on the ice block. The pressure lowers 
the melting point AT THAT PLACE. The string thus cuts into the ice. If the 
surroundings are not too warm, the cut will "heal" above the string 
because the pressure is no longer acting there.
Putting salt on the outside of the block also makes the ice melt at a 
lower temperature THERE. Heat flows from the inside of the block to the 
interface where the salt is. The inside is cooled thereby, helping to 
reseal the cut left by the string. 
The actual freezing point of salt water depends on the type of salt and 
its concentration. It is always lower than that of pure water.
Best regards
Werner Sieber