What is the best way to melt an alloy of gold, silver, oxygen and gas, or an electric furnace is better?
So, casting. This is a big subject in the world of metal art work, and there is a ton of nuances to it, but to keep it simple it’s all about getting a quality casting when you’re done. So what is a quality casting? Getting a piece (or pieces) that fills completely, is nice and dense - in other words without any porosity, oxidation (fire stain), doesn’t have any impurities in it (e.g. carbon), or “warts” and other surface imperfections.
That said, the recommendations I make here are based upon the methods I have used during my 45 years of casting experience working in a small shop. There are casting methods both low tech (low cost equipment) and higher tech (high cost equipment) that I haven’t used that do a great job.
I was taught initially how to do lost wax investment casting where the metal is melted using a torch fueled by either acetylene and compressed oxygen, or a propane/oxy set up. I have also used natural gas/oxygen. I have also used an electric melt furnace. They all work well, but which is the cleanest method in order to prevent porosity and oxidation? In order from cleanest to “dirtiest”: electric melt furnace, propane/oxy, natural gas/oxy, acetylene/oxy. Why?
It’s all about keeping your melt as clean as possible while melting and during your pour into whatever casting machine is used (centrifugal or vacuum). Rather than me giving a lengthy explanation here, I encourage you to look up what a reducing atmosphere is. An electric melting furnace uses this concept when melting the metal intended for your casting. The torch melt method can be very successful if you keep the reducing atmosphere concept in mind. With any of these fuel gases you need to oxygen in order to increase the torch flame temperature needed to melt the metal. If you decide to melt your metal using a torch, then you must use something that will create an oxygen barrier that will prevent the metal from absorbing the excess oxygen coming from the torch while melting/pouring it. Most metalsmiths use what I was taught to use: borax or some form of it like boric acid.
In addition there is carbon which comes from any of these fuel gases and can find its way into your melt. There are other things that can contaminate your metal as well, but carbon for sure will always be present in a torch flame also. And the fuel gas with the least amount of carbon in it is propane, and the one with the most is acetylene.
There is a lot to know about casting, but I hope this reply helps you make a decision about which was to go.
Good luck, Mike
I use an electric oven and I suffer from a lot of oxidation, but I don’t know what the reason is. That’s why I received this question. This is an example of what I suffer from
See the following:
I hope it helps.
Just out of curiosity.
How susceptible are high purity gold to firescale/oxidation.
I was of the impression that this pure gold should be quite resistant.
Pure gold don’t even need borax for melting and pouring.
And it is always clean and nice even in air.
Maybe there are some issues with the alloy?
What kind of alloy do you use?
I know, 21 karat, but is the alloy new to you, have you bought it as 21 or have you mixed/made it yourself?
An ingot of pure gold, silver and brass were added to it to become 21 karat, but when the ingot is melted in a gas furnace, it does not oxidize, and when it is melted in an electric furnace, it oxidizes, bearing in mind that the smelting in the electric furnace must be cleaner and safer.
Do you make the alloy yourself?
Is it tested in any way?
Edit to add:
Why brass and not copper?
Do you have full control of the quality of the brass?
12.5% Silver and brass
Is that right?
Yes, I purify the gold with my own hands and return it to caliber 999, and then I mix it with caliber. 21. 10% silver/2.5% Turkish brass yellow , but your question: did you test the alloy? I didn’t understand it well.
I was thinking that if you bought the alloy from others, if then you tested the quality.
Since you do things in house you should be in full control.
I just find it odd with that much firescale/oxidation on such a pure alloy.
Until the print professionals chime in I’d try to list up what you do here that is new.
And find info for these things.
If it was me, I would be pushing these print people until I got satisfactory information on the resin, maybe ask them to use wax, if its not wax already.
Same with the investment supplier.
Have you checked AurumArcanums advice on investment?
A little thing as a gentle air flow into the sprue opening at the end of the burnout, might solve a lot.
Do you have an automatic burnout kiln?
While casting alloy often contains a small amount of zinc to improve fluidity, unless the composition of the brass you’re adding is known and consistent across batches you’d have more control of the alloy by adding copper and zinc separately.
What’s the composition of “Turkish brass yellow”??? If that is unknown, then you can have no idea what you are alloying ! I would never alloy brass with gold–only pure elements…
Janet in Jerusalem
Yellow copper contains one third of zinc and the rest is red copper
I have melted using electric and with gas, (oxy/propane), eliminated acetylene several years ago due to the impurities in the gas.
Melting 24K gold will not oxidize, anything else will. (the alloys are coming to the surface), for alloying 14k I use pure gold, pure silver, copper and traces of zinc. I think it is 0.6%. Melting temp of zinc is very low so it will burn off as the other alloys are brought up to temp.
It appears as though your alloy has too much zinc which is coming to the surface as black oxidation. If not the zinc, there is something wrong with your alloy
There are literally hundreds of brass alloys (in some languages ‘brass’ translates as ‘yellow copper’). Are you saying that yours is 66.66% copper and 33.33%? And that you are certain it contains no other elements?
Yes, I am sure, because I have a German-made alloy tester. It gives me the percentages of metals with an error rate of 2 per thousand
It measures all trace elements in an alloy (not just the precious metals)?? Wow. Could you possibly give a link to such a tester? Thanks.
Based on your picture, I would say your biggest problem is insufficient spruing. The model does not fill because the metal can’t get to all parts before it solidifies. You have super thin details there, and the only way for the metal to fill the larger parts in between is trough those hair-thin parts. I would add more and thicker sprues, perhaps even a system of sprues in the back to every thicker part. Maybe run them to a thick ring-shaped sprue in the back. And definitely one proper thick sprue to the middle. And keep all sprues as short and thick as possible.
The sprues need to be thick enough to solidify last and the area the sprue supplies needs to solidify from the perifery towards the sprue. Othervise you will get porosity or in the worst case unfilled areas.
Your main sprue seems excessively long, the metal cools off before it even reaches the model. The easiest way to attach sprues to thin sections is to 3D-model short sprues (5-10 mm) in the right places, print them attached to the model and then attach the real sprues to those. That way you don’t have to melt sprues directly to delicate parts and you get clean transitions from model to sprue.
This is where I would start regarding your problems with this project. Oxidation should not be too much of a problem as you can easily get rid of it by pickling the casting, just like after soldering.
I’m no expert, this is just my five cents to this project that seems to be spread out over a riddiculous number of threads on this forum…