I was under the impression at the time that a gold furnace removed impurities and you were left with 999 fine gold. From what a gather now it just melts all the impurities into one big glob. Melting vs smelting .
You are so right! melting first then attempting to refine makes the task extremely difficult, if not impossible in some cases.
The correct procedure to achieve .999% quality (this is a condensed instructional that only briefly outlines the order o the steps involved, step-to-step procedures like, filtering, cleaning, re-filtering and such are not included) is to first chemically treat your mixed metals in Nitric Acid (using a fish tank bubblier to add aerate the solution speeds up the process). Once the base metals have been removed dissolve whats left in Aqua Regia. Once the gold is compleeted dissolved in the Aqua regia, filter solution to catch and remaining contaminates such as plastics, glass or other contaminates not affected by the Aqua Regia; To the remaining gold bearing liquid add a chemical (there are a few different choices of what chemical to use for this step so none have been quote, do the research to find out more) which bonds will force any gold in the solution to "drop out" and fall to the bottom of the container (looks like a brown mud once completely settled) Once all gold has been dropped, filter off the remaining liquid collecting the "mud" in the filter, wash a few times and dry. What you have left is 95 to 99% pure gold powder.
Mix this powder with the appropriate quantity and type of flux, put in your melting container and apply heat. The flux will "wash away" any impurities which may remain and all of the "mud" will melt together into a very pure gold ball in the bottom of the container. In order to take the gold to .999 quality will require, in most cases, that your repeat the procedure starting where you dissolve the gold in Aqua Regia and ending where you melt the "mud" together into a gold ball.
Again, I have only briefed over the major steps and steps like filtering, washing, filtering again, washing again and so on. melting the gold together with any base metal contaminates first only complicates the process making it more time consuming and more expensive to to achieve the goal of .999 pure gold