Flux to melt 24k to 18k gold

Hi.
I am melting 24k to 18k with copper and silver.
I melted copper and silver together first. It came out with irregular colour.
Then 2nd time, i melted gold and this batch together. I left little longer in the electric furnace like 20min.
Looks better. Still see gold and copper didn’t get on well.
Wonder Borax power is not suitable flux for this work? Or I just have to leave it longer inside furnace?
Anyone can give me a advise?
Thank you
Gina

I use borax with Gold and have not had any problems, may not be hot enough, I don't use a furnace so I tend to swirl the metal around this may help

Have you seen this article on melting your own alloys?