If a piece of copper is heated in an environment with no air (or any other means of heat transfer, such as conduction or convection), its temperature will continue to increase until it reaches its melting point.
Copper, like most materials, has a specific heat capacity, which is the amount of heat energy required to raise the temperature of a given mass of the material by a certain amount. As heat is applied to the copper, its temperature will rise as it absorbs the energy.
However, once the copper reaches its melting point, which is approximately 1085 degrees Celsius (1985 degrees Fahrenheit), the temperature will plateau and remain constant until the entire piece of copper has melted. During this phase transition from solid to liquid, the heat energy is used to break the intermolecular bonds holding the copper atoms in a rigid lattice structure.
Once the copper has completely melted, further heat input will cause the temperature of the copper to rise again as a liquid, following the specific heat capacity of the liquid copper until it reaches its boiling point.
It's important to note that this explanation assumes ideal conditions, where there is no other heat transfer mechanism present except the heating source. In reality, heat transfer through conduction may occur if the copper is in contact with a heat source, and heat dissipation through radiation may also be a factor depending on the setup.