From the Wikipedia article on fudge,
Are you trying to make a hard caramel?It is the endpoint temperature that separates hard caramel from fudge. The higher the peak temperature, the more sugar is dissolved and the more water evaporates, resulting in a higher ratio of sugar to water. Before the availability of cheap and accurate thermometers, cooks would use the ice-water (or cold water) test to determine the saturation of the confection. Fudge is made at the "soft ball" stage, which varies by altitude and ambient humidity from 235 °F (113 °C) to 240 °F (116 °C). Butter is then added to the mixture and the fudge is cooled and beaten until it is thick and small sugar crystals have formed.[2] The warm fudge is sometimes poured onto a marble slab to be cooled and shaped.