What preserves ice?

Temperatures below 32 deg f. In the old days and probably some places today what they did was, in the winter they would carve large blocks of ice from the lakes and rivers. They would take these blocks of ice to the barn or ice house, lay down a layer of saw dust and put the ice on it. Then then would put another layer of ice and then a layer of sawdust. They did this until the barn was full. The sawdust kept the ice insulated and it would last all through the summer until things began to freeze again.