The 18k rose gold made with only copper as the alloy can be a big
problem if it is not rapidly quenched after heating. It has to do with
the fact that the 75%weigtht gold 25% weight copper forms a ordered
rather than a disordered atomic latice this makes it hard and brittle.
If it is quenched from high enough temprature it will remain in the
disordered state but if it is allowed to cool slowly the ordered
structure forms and disaster is just around the corner. Most 18k red
alloys have a little silver or other metal addtions to reduce the
likelyhood of this happening but it is not as strong a red color. If
you are working with 18k red gold you really must quench it rapidly or
this will be a continuing problem. In 14k you can get away wuth a
copper gold binary alloy and it will not have as many problems but it
is still better to quench it.
Jim