Flux question.

[BillS]

I would like to say hello to everyone as this is my first post on this forum.

To harold_V I have read some of your posts about how you used to refine gold and other metals. I have a problem with the flux being to thick when melting gold. I use borax.

A friend asked me to melt some gold in my propane furnace because his furnace using coke was very slow and messy.

I know that the melting point of pure gold is very low, so my furnace would melt it very quickly. (4 mins.) I noticed that as the melt progressed the flux got thicker because it is dissolving the crucible wall and was very dark. I am using a clay graphite crucible. When the contents of the crucible was poured into the ingot mold all the gold came out but most of the flux stayed behind in the crucible.

Do I have to add more borax just before I pour or do I have to add soda ash to thin up the flux?
This is the first time I have melted gold, it was a learning experience for me.

In the past I have used bottle glass as a flux for brass but found it was too thick so I added some soda ash to make it thinner.

Bill.

[Harold_V]

Bill,
Soda ash will modify how the flux behaves, and may lend to problems rather than solve them. It all depends on your objective.

You didn't mention what your purpose was in melting the gold (although you did allude to it being pure, and that you were pouring an ingot), but understand that adding soda ash will tend to reduce oxides of other metals, re-introducing them to the charge. That may or may not be an issue for you. However, soda ash will be all the harder on your crucible. I know of no flux mixtures that will be user friendly in that regard. Even my 94% alumina furnace didn't like flux being added, although it was a necessary evil.

If your purpose is strictly to thin the flux, you might consider adding a small amount of fluorspar. I used it when processing waste materials (in the 94% alumina furnace), with excellent results in thinning. It takes very little to create water thin flux. However, like soda ash, it, too, will be hard on crucible life.

I used to melt silver (in powder form, known as cement silver, recovered from solution, nearly pure, but containing traces of copper), using borax (added sparingly). I could count on only eight heats, using a silicon carbide crucible, before it was perforated at the flux line. Below the line the crucible was in near new condition. For clarity, I used #8 crucibles in that furnace.

The very best scenario is to melt with little to no flux. Pure gold needs no flux, but alloyed gold requires some, as the base metals oxidize when heated. If you can detail a little better what you're melting, and what you expect of the process, I may be able to provide a somewhat better suggestion. Pure gold can be melted without issue, but I never used a furnace for the processes, even when I had a few hundred ounces to melt.

By the way, gold does not melt at a low temperature (unless alloyed). Pure gold melts at about 1950° F, about the same temperature as copper.

Harold

PS---Welcome to the board!

[BillS]

Harold

Thank you for your answers. The gold came from ore which was crushed to extract the gold. I would say that it would be alloyed with silver and other metals and would have a melting point lower than pure gold. My friend has no intention to refine it into pure gold, it is to have a large lump of gold for the wow factor.

As my friend liked my furnace so much he is going to copy and build one so I will recommend to my friend to get some fluorspar to thin out the flux.

I use my furnace a lot to melt cast iron for model engine parts. It can a while to melt cast iron but the gold melted very quickly. The difference between the melting points of gold and cast iron is about two hundred degrees why does it take longer to melt cast iron?

Bill.

[Harold_V]

Harold

Thank you for your answers. The gold came from ore which was crushed to extract the gold. I would say that it would be alloyed with silver and other metals and would have a melting point lower than pure gold.[/quote
Armed with that information, I'd likely recommend that some soda ash be used (it deals with silica), and a bar of iron (some scrap steel, like a piece of angle or channel, or even a length of rebar) introduced to the heat. Assuming the ore was a sulfide, iron that is introduced will liberate the values, as it will readily combine with any sulfur that may be present. You can generally identify sulfides, as they form a layer between the flux and metal when you pour. It tends to have the appearance of cast iron. Assuming you pour to a cone mold, the values will form a distinct layer (the button), then a distinct layer of sulfides, then the flux, again, a distinct layer.

My friend has no intention to refine it into pure gold, it is to have a large lump of gold for the wow factor.

Understood. Not many can claim to have made such a recovery.

Can you define the color? The surface is most likely to be far different from the interior, as there is likely to be some oxide formation coating the resulting metal. Take note that it takes very little silver to swing the color of gold away from yellow. In fact, if the predominant alloy is silver, you can expect the gold to assume a green cast, although not so much if there's copper present.

I use my furnace a lot to melt cast iron for model engine parts. It can a while to melt cast iron but the gold melted very quickly. The difference between the melting points of gold and cast iron is about two hundred degrees why does it take longer to melt cast iron?

Now I understand why you suggested that gold melts at a low temperature. Because it's alloyed, yes, it melts at a low temperature, although that's a relative term. How low can be described only by knowing what it contains, and the ratios of the elements, but you can expect that gold alloy will melt in the vicinity of 1,750° (assuming there's no lead present), while cast iron will melt at around 2,400° (just generalizing---don't take these numbers to the bank). The lower the carbon content, the higher the melting point, at least as I recall. Crucible furnaces tend to be poorly insulated, and may or may not be fired by fuel that provides the needed BTU's, thus, in general, not well suited to melting iron. There are also issues with chemical changes in the heat, due in part to the crucible, as well as the prolonged heating time. Some seem to take offense when that is mentioned, but the fact remains that it is not easily accomplished. You're fortunate to have worked out the kinks, so you can enjoy success.

Harold

[DavidF]

I use my furnace a lot to melt cast iron for model engine parts. It can a while to melt cast iron but the gold melted very quickly. The difference between the melting points of gold and cast iron is about two hundred degrees why does it take longer to melt cast iron?

Congratulations on melting iron!! Would you mind telling me more about your furnace, im looking for the inner dimensions, vent size, crucible size and burner type. Also interested in if it is lined with solid refractory or a hot faced K wool.
From what I have been noticing there is a cut off point for the BTU's propane can deliver and the volume of the furnace. Im trying to find that break point. Harold hit on a key point, that various compositions of iron have different melting temps, and I doubt I can hit the temps required to melt pig iron in my small furnace, but have no trouble melting a 40 class grey iron.

[BillS]

Harold

Sorry I took so long to reply as I lost the piece of paper that had the forum password written on it.

The gold has a nice yellow color before it is melted but after it is poured into a ingot mold it looks like brass. It must have a lot of silver alloyed in the gold. When gold cooled down in the ingot mold it had a discoloration where the flux did not cover it. A pickle solution was used to clean the gold to a bright shiny surface. I will have to ask my friend if it is a sulfide ore.
Most cast irons melt in the range 2100- 2250 and are very close to the eutectic point (4.3% C.E.) this to keep shrinkage problems to a minimum. When graphite forms when cast iron cools it expands in volume to counter shrinkage. Remelting cast iron does not alter the carbon content that much and because it is close to the eutectic point it will absorb very little graphite from the crucible walls. To prove that point melt some cast iron and add any form of carbon such as charcoal, coke or a graphite rod from a battery. It will just float on top and after a very long time may absorb a tiny amount of carbon. If you go over the eutectic point, the melting point will skyrocket so it causes other problems. You are right a crucible furnace is not a suitable furnace to melt cast iron so I work around these problems to get the castings I need.

DavidF

I was very amused by your statement < and I doubt I can hit the temps required to melt pig iron in my small furnace.>

Can you please enlighten me why pig iron cannot be melted in your furnace? Have you actually tried to melt pig iron in your furnace?
There are two kinds of pig iron, one is made in a blast furnace and the other is made in an electric furnace. I have melted both types in my furnace with no problems getting up to temperature. The pig iron that was made in a blast furnace I used in a 50/50 mix of pig iron and disc brake rotors, it made some of the best castings I have ever made. It was so good that no ferrosilicon was needed to make the castings machinable, the down side was cutting it into chunks that would fit in the crucible. The sad part is that I can't get any more because foundries use scrap steel and raise the carbon content to cast iron level. It is cheaper than pig iron. The electric furnace pig iron is to be used for ductile iron production and when I used it lots of ferrosilicon was needed to make the castings machinable. So now I use scrap cast iron that was carefully selected for its ease of use in the backyard furnace.
You mentioned melting class 40 grey iron in your furnace, how do you know it is class 40 grey iron? Do you buy round and square bar with a known pedigree?
A class 40 grey iron has more tensile strength than than a class 20 grey iron but that does not mean that there is a huge difference in the melting point. With my furnace I have not noticed a cut off point on how much Btu's that can be delivered in my furnace. If I need more Btu's I speed up the blower and add more propane. Theoretically there is no limit to how fast the blower can run but having such a fast velocity in the furnace can cause other problems. Having very good furnace insulation helps to melt cast iron faster than having the blower running faster and it also uses less propane.

Furnace details, 7" Dia 9" high, vent 3.5", crucible size A3 clay graphite, forced air propane burner, hot face is kaowool.

Bill.

[Harold_V]

The gold has a nice yellow color before it is melted but after it is poured into a ingot mold it looks like brass. It must have a lot of silver alloyed in the gold.

It's a little more involved than that. Silver, in excess, will shift the color of gold to not white, but green.

Yeah, I know, that makes no sense, but then it makes no sense that copper is the color it is, but is blue when dissolved in nitric acid.

But there's more to this than meets the eye---

When gold cooled down in the ingot mold it had a discoloration where the flux did not cover it.

Indications that the alloy contains something beyond silver, as neither gold or silver will oxidize when melted. The discoloration is the oxide of copper, usually, and that may explain why the material had a brass color instead of a rich yellow color of gold.

A pickle solution was used to clean the gold to a bright shiny surface.

That's what jewelers do after heating gold alloys. The fire coat is expected, and is removed accordingly. Most jewelers use a commercial product called Sparex (reputed to be safer than sulfuric), but dilute sulfuric acid works perfectly well. Copper does not dissolve in dilute sulfuric, but copper oxide is readily dissolved.

I will have to ask my friend if it is a sulfide ore.

It would be in his best interest to use scrap iron in the heat if it is. He'd most likely get a better recovery.

Most cast irons melt in the range 2100- 2250

Thanks for that bit of information. I've quoted a higher temperature, which most likely would be required for pouring. I'd appreciate your comments in that regard.

Remelting cast iron does not alter the carbon content that much and because it is close to the eutectic point it will absorb very little graphite from the crucible walls.

While it may not absorb much, it has been my impression that cast iron that is heated too slowly is troubled more by carbon being eliminated, as it is converted to CO2 (resulting in white iron). I also recall that the amount of carbon contained in gray iron is controlled by the amount of silicon (Ammen's book, as I recall). Your comments in that regard would be appreciated. It's been a long time since I read on the subject, so I may have it wrong.

You are right a crucible furnace is not a suitable furnace to melt cast iron so I work around these problems to get the castings I need.

It is my opinion that crucible furnaces would be used by small foundries to pour iron if they were a proper fit for the purpose. Considering that is not the case lends credence to the notion that they are not a good choice for that purpose. I appreciate your candor in support of that view, and my hat is off to your for working around the issues that make the process functional for the home foundry. Sure beats not having the capability.

Thinking about your comments about how it is harder (takes longer) to melt iron than it did this gold alloy---it makes sense. The temperature difference is likely more than 300°F, which is a large amount to make up when crucible furnaces are not known for achieving the higher temperatures (there are exceptions). I noticed a considerable difference when melting pure copper as compared to silver and alloys of gold, which are melted together for refining. The temperature for copper was much higher, made obvious by the internal color of the furnace, and the prolonged heating period. I think I understand!

Harold

[DavidF]

I was very amused by your statement < and I doubt I can hit the temps required to melt pig iron in my small furnace.>

I see why you are amused. I live on a piece of land that was first patented in 1650 to a couple of sweedish familys and I often explore thier settlements and research the history of the area. A couple of very interesting things have come up. At first glance this area would seem worth little more than good farm land but research has shown there was much more going on than farming. At one time they were building boats here (seemingly insignificant ) But they were also forging and smelting iron to produce wrought iron. The iron ore hematite came from an area near by "Iron hill" and was smelted using charcoal. I think thats pretty cool myself. There is a creek that runs here named "white clay creek" that has a very pure kaoline clay lining its bed. This clay was so highly revered by the british that they actually forbade the colonials from making thier own whiteware from it. Instead it was to be shipped back, made into dishes or what ever then shipped back so it could be taxed by his royal hiney. I have rolled this clay into a ball and let it dry, then hit it with a oxy acetyline torch to near white hot without it breaking down (impressive stuff). At some time with the available resources I would like to try and pig iron just to see if I can and if it possibly was done. So when I say i doubt I can hit the temps to melt pig iron, I really meant that I doubt I could hit the tempatures to pig iron which I think is about 2900'F?? Correct me if im wrong, Im still learning here...

You mentioned melting class 40 grey iron in your furnace, how do you know it is class 40 grey iron?

A friend of mine and I built our first foundrys about the same time. We both talked about what we were doing and the things we had melted. We both wanted to be able to cast iron, but my friend (who worked in a steel mill) said that it was not possible to melt iron in a crucible furnace, especially when powered by propane. There was just no way in his mind it could be done. I admit I tried several times to get it to melt and only wound up with a pile of goo in my crucible to show for it. But then I did get it to melt, made the toughest white iron you could think of, but it did melt. Later on I found a good supply of iron that I could melt easily and repeatably, and with a small addition of ferro silicon produced some very nice and machinable castings. My friend was astonished but still in question. He then had me pour a couple of thin sticks (paint stir sticks) and took them and a hunk of the scrap back to work to assay them. The scrap was a 40 class grey iron as well as the first test stick that had about a 1% FeSi addition. The second stick which I poisoned the melt with FeSi fell into a 30 class iron range. Now im not metalurgist and am relying on what I was told to be true, but none the less I was perfectly happy with the resulting castings.

You are right a crucible furnace is not a suitable furnace to melt cast iron

Now this I just dont understand. I can see that if you are trying to produce an exacting alloy then it would not be acceptable. But you seem happy with the castings you have done, and would likely do it again. So what exactly is not suitable? Obvoiusly Im missing something here, but to me "if it works it works". It may not be up to certain standerds, but it is within the realm of the back yard hobbiest, and thats the point im trying to drive home.

So now I use scrap cast iron that was carefully selected for its ease of use in the backyard furnace.

Yes I see that myself, but have no good way to really identify what is going to work out better, from one iron to another. I test drill my iron with a 1/4" drill bit to see how easily it drills and then crush the chips between my fingers to see how easy they crush and how black my fingers are from doing it. Very unscientific but does give some insight and has so far worked out for me. Have any tips or advice you can offer?

I have not noticed a cut off point on how much Btu's that can be delivered in my furnace. If I need more Btu's I speed up the blower and add more propane.

So many things to think about here. Furnace design being a key role. Evaporation rate of propane also to be considered. I would really like to gather more information before derriving at a conclusion. There has been some that have not been able to melt iron, and some that have. So what makes some so successful and others saying it cant be done? I am certain that there are alot of individuals out there who would love to melt iron, but when they see failures and negativity on casting iron in a crucible furnace will shy away from making the attempt for themselves. There is much to learn, but it is possible...

Furnace details, 7" Dia 9" high, vent 3.5", crucible size A3 clay graphite, forced air propane burner, hot face is kaowool.

My furnace is a 6" bore 8" deep solid refractory (ap green 3200') 3" vent, 3/4" reil burner .040" orfice, forced air by wifes hair dryer, and an A6 clay graphite crucible that from cold will melt 10 lbs iron in about an hour. There is very little room for lifting tongs in my furnace with this crucible size but it seems to work well.
You say your hot face is Kaowool? Could you please elaborate on your furnace lining..

[BillS]

Harold

Now that you mentioned the different colors of copper salts some are green in color as well as blue. I have read that if 20% aluminum is added to pure gold it results in a purple color.

You mentioned that 2400 F would be the pouring temperature, yes it could but is a little bit low for small crucibles. I pour at 2600 F as it gives enough time to pour more than one mold before the iron cools down to much. I have a R type pyrometer so it saves a lot of the guess work.

Here is another experiment you can try: Melt cast iron with a layer of carbon floating on the molten metal to prevent carbon loss by oxidation then pour. Melt another crucible full without carbon floating on top and add ferrosilicon then pour. The iron that had ferrosilicon added will always be softer than the melt that had a carbon layer floating on top. The reason why this happening is when cast iron remelts the innoculation effect is lost, not because carbon has been lost. Ferrosilicon is always added just before you pour because the effect fades with time. When I first started using it ferrosilicon it was added at the start and I wondered why it was not working. How does ferrosilicon work? When cast iron has been remelted it is a mixture of grey and white iron so when ferrosilicon is added it provides seeds for graphite to grow and a soft iron is the result. Having said that if carbide stabilizing elements are present a lot more ferrosilicon has to be added, sometimes it will not work because there is too much carbide stabilizing elements present. If too much ferrosilicon is added then it becomes a carbide stabilizing element as well and adding too little it has little effect. Trying to dissolve carbon after adding ferrosilicon will show you how effective ferrosilicon is. Yes you are right carbon content is controlled by how much silicon is present in cast iron.

There are two good reasons why small foundries do not use crucible furnaces to melt iron.

1. Crucible sizes are generally too small for most foundries so ladles can be made to any size by a small foundry.

2. Crucible furnaces are expensive to run compared to other furnaces. When a fuel is burnt in a crucible furnace it has to heat up furnace walls, heat up the nitrogen in the combustion air, heat up crucible walls and what is left over heats up the iron to the pouring temperature. I have read that only 5% of the fuel burnt heats up the metal being melted. You mentioned that crucible furnaces are not known for achieving the higher temperatures (there are exceptions). So many backyard melters throw together a furnace that will melt nonferrous metals, but when they try to melt cast iron it nearly always ends up in failure. To get higher temperatures more fuel and air are needed plus very good furnace linings are needed to slow down heat loss.

The problems I work around to melt cast iron are: crucibles don't last a long as melting nonferrous metals and the same thing happens to furnace linings. It is still cheaper to pour my own castings than to drive three hours to the nearest foundry to get them to pour one off's when they are not interested in the first place. You just can't beat pouring castings when you want to and not having to rely on other foundries.

DavidF

Wrought iron is not pig iron.

Unless tensile tests were made on your test pieces you cannot say it is class 40 iron

My furnace will melt iron but it does not make it the ideal furnace to melt iron.

Scrap iron that was made from pig iron works well for backyard melters

Btu's for furnace, Read my post above to Harold

Kaowool hot face with a zirconia paint to protect the fiber from flame erosion

Bill.

[Harold_V]

Bill,
Various colors of gold alloys can be created, although some are not ductile in the least, so they must be cast in the desired shape, with no stones mounted.

As you alluded, aluminum does yield purple, and iron yields blue. The colors are never vivid, but they are bright enough, for lack of a better description, to be recognized as the color they represent.

White gold is typically comprised of nickel and gold, but palladium and gold will also work. In all my years of refining, I rarely encountered palladium in white gold, however, but a conversation with an old friend (a manufacturing jeweler) disclosed that it is far more common today, the result of many having an allergic reaction to nickel..

Thanks for the further comments on carbon content in gray iron. At this point, all I can do is discuss the issue, as my shop is not functional, but that is going to change in the near future. I've waited (patiently) for years to be able to experiment with foundry practices. I hope to get my induction furnace operational soon, which will allow for rapid melting and experimentation, typically without altering chemistry of the heat. Perhaps you and I can further discuss these issues at that time, so I can do some experimenting in real time, gaining knowledge for the future. Mean time, thanks for sharing your hard earned knowledge with me.

Harold

[DavidF]

2. Crucible furnaces are expensive to run compared to other furnaces. When a fuel is burnt in a crucible furnace it has to heat up furnace walls, heat up the nitrogen in the combustion air, heat up crucible walls and what is left over heats up the iron to the pouring temperature. I have read that only 5% of the fuel burnt heats up the metal being melted. You mentioned that crucible furnaces are not known for achieving the higher temperatures (there are exceptions). So many backyard melters throw together a furnace that will melt nonferrous metals, but when they try to melt cast iron it nearly always ends up in failure. To get higher temperatures more fuel and air are needed plus very good furnace linings are needed to slow down heat loss.

Well what othert choice does a back yard hobbiest have? To me a crucible furnace is the least expensive way to melt iron for the home hobbiest. If you know of a less expensive way then do tell.

crucibles don't last a long as melting nonferrous metals and the same thing happens to furnace linings

Honestly I have seen little difference in the life of a crucible from melting iron or silicon bronze. Both of which I get about 10 melts befor replacement is necessary. I am using a budget clay graphite A6 crucible. Furnace lining has been holding up well but is starting to need the floor redone. I could not even begin to make a guess at how many melts are on this furnace but im very happy with its lifespan.

Kaowool hot face with a zirconia paint to protect the fiber from flame erosion

Hows it holding up so far? Pictures?

[BillS]

Only 10 melts? I get between 35-40 melts. I would never use budget crucibles they are really badly made.

The kaowool lasts for about 25 iron melts. I get a lot of kaowool given to me so cost is not a problem so why should I use anything else.

Bill