Non-Chemical Pouring exactly 1 troy ounce at a time

[Harold_V]

Tim,

I've quietly followed your exploits for some time now. Congratulations on your apparent success with the induction furnace. Very nice!

Harold

 

[MargueriteMingorance]

...
Okie doke. So, need to use something other than graphite, or metal, for the mold, right? What about fused quartz? That's like glass, right? It should give a very smooth finish.

Since you're reheating the gold in the mold, you can bring the temp up slowly, and then let it cool slowly. This would avoid thermal shocks. After all, that is how glass is anealed. Since the gold will shrink on cooling, it should come out of the mold fine. A mold made of SiO2 won't contaminate the gold, will it?

For silver, could you coat the mold with a dusting of graphite? It would burn away as CO2, absorbing any oxygen. I would suggest oil, but it contains impurities, like sulphur and chlorine, and maybe trace metals.

The melting point of pure SiO2 is 1650C. Melting point of gold is 1065C, silver is 962C.

Not a terrible idea, but a few points:

1. Quartz will tend to devitrify and decay at elevated temperatures after repeated use.

2. Machined fused quartz is very expensive.

3. Fused Quartz is very brittle and is easily broken if mishandled or dropped. The good thing about quartz is it can go from hot to cold relatively quickly and not shatter like Pyrex or lesser glass.

4. Fused quartz requires special handling considerations to maintain it's continued integrity.

Here's a usage guide for fused quartz:

Fused Quartz Usage Guide

I can't help but think you would quickly ruin your expensive quartz mold with skin oil, furnace trash, and other contaminates found in the metal and melting process.

If you could over come and/or address the points above, I think you may be on to something, but I've never tried it my self. I own several pieces of quartz ware and they require special attention unless you have deep pocket for buying new quartz equipment.

The expense of quartz fabrication will typically equal or exceed the already high cost of the raw materials to make the item out of quartz, in your case bar molds. If you have a few extra hundred bucks laying around and want to give it is shot, I for one would like to see the results.

Steve

Well, you have a furnace, right? Quartz is glass. So, get a mold made of the quartz mold you want, and buy pure SiO2 sand. Melt a slug of it, and form it in your own (cast iron or steel or whatever) mold. Then you can make all the quartz molds you want. Fire polish the fused quartz (which seems like a fancy term for glass, to me) so you have a smooth finish. Why pay someone else to make your own disposable molds???

Plus, if your quartz mold breaks, just remelt it.

EDIT: turns out fused quartz is just a fancy name for fused silica. Like refining, it is one of those things where it is hard to find information on doing things yourself.

As for making a furnace with an inert atmosphere, that seems easy to me. Get a furnace whose top and doors are air tight. Add some fittings into the bottom (2). One fitting you use to inject helium, which you can buy in tanks. The other is to let interior air out the bottom. Helium is light, it will rise, and force the heavier air out the bottom of the furnace.

 

[jimdoc]

I don't think it is that easy,they would not cost as much as they do if that
could be done easily.

Jim

 

[Harold_V]

Well, you have a furnace, right? Quartz is glass. So, get a mold made of the quartz mold you want, and buy pure SiO2 sand. Melt a slug of it, and form it in your own (cast iron or steel or whatever) mold. Then you can make all the quartz molds you want. Fire polish the fused quartz (which seems like a fancy term for glass, to me) so you have a smooth finish. Why pay someone else to make your own disposable molds???

Plus, if your quartz mold breaks, just remelt it.

It appears to me that your post is one of antagonism, not intended to be constructive in the least. Please refrain from such ativity on this forum.

Your comments are not relevant. The melting point of quartz is nearly 1,000°F above the melting point of cast iron. It is roughly 500° above the melting point of steel. Needless to say, your comments have no value, as they simply won't work.

Harold

 

[MargueriteMingorance]

Well, you have a furnace, right? Quartz is glass. So, get a mold made of the quartz mold you want, and buy pure SiO2 sand. Melt a slug of it, and form it in your own (cast iron or steel or whatever) mold. Then you can make all the quartz molds you want. Fire polish the fused quartz (which seems like a fancy term for glass, to me) so you have a smooth finish. Why pay someone else to make your own disposable molds???

Plus, if your quartz mold breaks, just remelt it.

It appears to me that your post is one of antagonism, not intended to be constructive in the least. Please refrain from such ativity on this forum.

Your comments are not relevant. The melting point of quartz is nearly 1,000°F above the melting point of cast iron. It is roughly 500° above the melting point of steel. Needless to say, your comments have no value, as they simply won't work.

Harold

Well, my tone was intended to be light-hearted, not insulting, and my intent was to be constructive.

I don't know what glass makers make their molds from, but they do make them from something, that can withstand the heat. Find out what they use, so you can make your own fused quartz molds. Just because I was wrong about what substance to make the molds from doesn't mean my comments have no value at all.

Of course, you could use a fired ceramic mold, but glazes often have metals in them, and an unglazed surface wouldn't yield a very nice finish on your exactly one ounce (or any other size) ingot.

Here is a site that sells books and glass making supplies, including stuff to make your own molds to cast your own glass:

http://www.sundanceglass.com/castrefractr.htm

Or, get your glass mold make from tungsten, its melting point is 3420C. Items made of tungsten are typically sintered. Finding an american company to make such a mold seems difficult, but Chinese manufacturers are all over the place. Molds seem to run in the $100-1000 range. They make molds for injection molding of metal.

Chinese industry is working hard to beat the pants off the US.

 

[Harold_V]

Well, my tone was intended to be light-hearted, not insulting, and my intent was to be constructive.

I'll consider that that is the case.

I don't know what glass makers make their molds from, but they do make them from something, that can withstand the heat.

They often use cast iron. The problem is, you don't appear to understand that fused quartz isn't the same thing as glass, including borosilicate glass. Common glass melts at temperatures under 2,000°. Assuming you have the ability to melt quartz, it will be liquid at a temperature in the vicinity of 3,300° F, which would have, long ago, melted the vast majority of metals.

Just because I was wrong about what substance to make the molds from doesn't mean my comments have no value at all.

You appear to me to be an individual that gets an idea, puts his head down and plunges forward, ignoring anything that may not be to his/her liking.

Understand that working with precious metals is an old art---very little is new. Those that have gone before us have discovered what works, and what does not. It's highly unlikely a guy/gal (not intended to be a personal affront) that has no history of working with precious metals is going to make a discovery that revolutionizes basic processing. One is far better served to follow convention, at least until a firm understanding of the problems are at hand, instead of second guessing those that have years of experience and understand the problems well.

Of course, you could use a fired ceramic mold, but glazes often have metals in them, and an unglazed surface wouldn't yield a very nice finish on your exactly one ounce (or any other size) ingot.

You're assuming a great deal here. Lets assume, just for a moment, that you can make a ceramic mold, and I'll consider that it may not have to be glazed. Will it have the required thermal shock resistance to withstand having a substance that is in excess of 3,300° F poured to it and not self destruct? Can one, in fact, pour molten quartz? I don't know that it can be poured, and likely can not.

Harold

 

[MargueriteMingorance]

Actually, fused quartz is glass. "Fused quartz" and "fused silica" are basically interchangeable. It is pure SiO2, which is a fairly high temperature glass, with high thermal resistance because it has a very low expansion coefficient. From what I have been reading over the last hour, it seems to be fairly durable, although it does degrade with time at high temperature, and is brittle.

Glass manufacturers add things like boron and other trace elements for color, hardness, and to lower viscosity (glass doesn't actually melt, as it doesn't have a crystalline form). What you call "common" glass is actually a fairly new invention. The ability to mass produce window glass didn't really happen until the mid-1950's, when the techninque of floating the glass on a bed of molten tin to allow the glass to slump to a flat heated surface became the norm. Before that glass was "poured" onto a metal table, where it slumped and cooled. This left the glass thicker in the middle than at the edges. Before that, it was blown into cylinders, then the top and bottom was cut out and the cylinder slit and opened to make flat panels. It is modern glass making that has mastered adding different substances to control viscosity, making glass more malleable at lower temperatures. Boron doesn't actually affect the viscosity, it protects glass from chemical attack, making it more durable.

Typically, formed glass is either slumped into molds, or a batch of it is heated into a "slug", which is then pressed into a mold. If the silica material is placed into the mold and then fired, it slumps into the mold, so there is no problem with thermal shocks. Glass molds are made from a wide variety of substances, including iron and steel, ceramics, cement, plaster, and various non-refractory metals.

As for putting glass into a ceramic mold, again, you can either slump the glass into the mold with the furnace, heating them together, or you can mold a slug. However, thermal shock should never be a problem, because you can always heat the mold ahead of time.

Here's a bit about different mold types for glass:

http://www.warmglass.com/molds_for_slumping.htm#clay

Here is a company that makes custom glass molds:

http://www.koppglass.com/Mold-Making-and-Design.html

BTW, fused silica can be sintered at less than 1300C:

http://www.faqs.org/patents/app/20080196448

Sintering takes time, a lot longer than it would take the gold for an ingot to melt into the form. As soon as the gold is melted the form can be removed from the furnace and allowed to cool. The gold will shrink, the silica mold won't shrink as much. Pure silica handles temperature transitions well.

 

[Harold_V]

And how much money are you prepared to procure this mold of sintered silica?

The point I've tried to make all along is that your idea isn't viable. You can toss all the information at me you wish, but the fact remains that such molds are not used for this purpose, nor is it likely they will be. If for no other reason, they are very expensive (checked price on a fused quart dish lately?), and can't withstand the mechanical abuse they would experience in a gold pouring facility. Worst of all, they don't offer anything that can't be accomplished by other means, at far lower cost. Sort of reminds me of gold plating the bumper of a car when chrome would serve equally as well.

The links you provided are interesting, and should prove to be useful for others. For that, thanks.

Harold

 

[T3sl4]

Ease up, Harold. Ignorance isn't to be punished. It's a treatable thing. Just because an idea, at face value, is trivially unworkable, doesn't mean it can't be a starting point to something that is. The obvious step is replacing a metal that doesn't work (like cast iron) with one that might (like tungsten), though I wonder if fused quartz sticks to it (which would be another problem entirely!).

OP: realistically, you're unlikely to do anything with quartz, even bend tubing, let alone make your own shapes. Work requires an oxyfuel flame, usually hydrogen or methane, though acetylene works in a pinch I believe. Something that hot cools down very quickly, and even with a high pressure injector, you may not get a ball of quartz into your mold fast enough to fill it out.

Sintering is interesting, especially with sintering aids, like flux, cement, clay, etc. The downside is, quartz remains quartz, so you still get quartz shock at 573C. Since silica is a big component of pottery, this is one of the limits on the firing rate of common ceramics.

The best, easiest to make materials are mullite (a mixture of alumina and silica) and cordierite (magnesia, alumina and silica, with a somewhat lower melting point and much lower CTE).

Materials that haven't been mentioned yet are pyrolytic and glassy carbon. Commercial graphite is porous; these may perform better, especially with an inert or vacuum atmosphere.

Tim

 

[MargueriteMingorance]

People seem to have gotten confused about the process, here.

1) get a custom made tungsten mold, or use ceramic or something else (a number of possibilities were listed in the links above). Ceramic is cheap, you can buy the stuff for $20. A custom tungsten mold is a bigger investment. Judging by the prices posted by some of the companies that make them, expect to spend $150-500 for it.

2) use the mold to form fused silica molds for ingots. This way, you can make your own reusable molds, if you have a furnace that is hot enough to work glass. 1300C seems to be sufficient, if you give the viscous glass enough time to slump into the mold. Glass isn't a solid, it is amorphous. At room temperature it is just very stiff. Of course, you're not going to do this sort of thing with a torch, you need an actual furnace with a thermostat, you need some actual control.

3) Use the glass molds to make your gold ingots. This way, you can make your own fused silica molds cheaply, so when they wear out, you can just recast them. Sand is cheap, just heat it up high enough.

4) When making an ingot that is exactly one ounce, you use powered gold, and measure it out into your fused silica mold. You put the whole thing back in your furnace, heat it up to 1100C (melting temp for gold) and let the gold melt. When the gold has melted, take it out, and use a torch to keep the top smooth until it cools.

BTW, one of the properties of fused quartz is it is very resistant to thermal shock, because it has a very low coefficient of thermal expansion. But even if your mold cracks, just make another one.

There is no need to pay someone else ridiculous prices to make fused silica molds, the process isn't that hard. If you wanted to do something like extrude tubing, there you would need some more specialized equipment.

This way, you can make ingots of an exact weight, every time. It solves the pouring problem, because you don't pour. You let the gold melt within your fused silica mold, which has a melting temp of 1700C (more or less, since glass doesn't exactly melt, it becomes less and less viscous, but at 1700C you can pour it). Make sure your silica is pure. If you get any alkali impurities in it, it lowers the viscosity. When you go to melt your gold, your mold might puddle.

BTW, with an inert atmosphere in your furnace (like helium) you can use graphite molds. They won't burn without oxygen.

 

[MargueriteMingorance]

Interesting article from the DOE's Office of Scientific and Technical Information.

http://www.osti.gov/bridge/servlets/purl/932641-trnH1t/932641.pdf

They were experimenting with fused silica molds to cast stainless steel parts. They like it, because with the low CTE, the mold doesn't expand much as it heats, so the parts produced are more accurate.

Here is a doctoral thesis on the crystallization of fused silica during sintering:

http://deepblue.lib.umich.edu/bitstream/2027.42/61652/1/baecj_1.pdf

The goal of the paper was to use sintered fused silica molds to cast parts for jet engines at temperatures as high as 1600C.

Fused silica is the best candidate for non-reactive alloys
fulfilling the restrictions on refractory ceramics used for investment casting such that it
has thermal stability at high temperature resulting from a low thermal expansion
coefficient (about 0.6x10-6/oC) and excellent thermal shock resistance. In addition, a
fused silica core is easily removed due to the complete chemical leachability in aqueous
solutions such as NaOH and KOH, where the solutions are non-deleterious to the nickelbase
superalloys.

BTW, sintering is technically interesting, because it naturally makes very smooth surfaces. It is one of its inherent characteristics. Sintering works by reducing the sintered material to a lower energy state--one with less surface area. A smooth surface has less surface area than a rough one, sintering naturally smooths things out.



For extra credit, how do you sinter a perfect sphere?

 

[qst42know]

I would guess producing fused quartz is out of reach for most. Unless you have a high pressure high vacuum electric furnace in your garage. However it's not out of reach for the General Electric corp.

The second page of this article has a good though brief summary of the process of producing fused quartz.

http://www.time.com/time/magazine/article/0,9171,718367-1,00.html

And don't forget the required annealing process.

http://online.momentivequartz.com/en/pdfs/annealing.pdf

 

[Lou]

I may wager I'm the only person here who has worked or even melted things in quartz.

Quartz is amazing stuff. It's not as expensive as everyone seems to think it is. I have a few things out of it.
Quartz molds and boats are available out of the catalog.

Oxymethane is unsuitable for all but the smallest things. Oxyacetylene works better and better yet is oxyhydrogen.

Of all the posts, Steve's seems the most knowledgeable--it's very easy to contaminate, and the salts from your fingers quickly aid in devitrification (whitening).
This is more of a cosmetic issue. Anyway, it is a great mold material. I've melted plenty of material in it and run reactions in quartz ampoules.
Quartz is formed around graphite tools and dies--graphite is noncontaminating as it will simply burn off.

To everyone here: forget about working it. There are only a small number of people who can make apparatus of it. I've only ever made ampoules and graded fittings and a few metal--glass seals.
It's very hard, hot, and dangerous to work.

 

[dtectr]

sound like some ""prospectors"" we've dealt with here? (real prospectors will get what the "double" quotes mean)

 

[Harold_V]

It's pretty simple. "Clever" people will have no part of having their opinion taken from them.

Had MargueriteMingorance spent any time in the trenches, facing the real problems that he appears to think he can address, he/she might see these issues from a different light.

When one melts gold powder, the beginning weight and the ending weight is generally not the same.

Why?

Because the best washed gold powder still contains traces of something that isn't gold, and it tends to be reactive when heated. It is common for miniscule balls of gold to fly from a lot of gold powder being melted. It may be small, but small is all it takes.

I'm having a hard time understanding how an individual that appears to have no experience in the field of casting gold appears to have so many opinions, none of which are commonly accepted by those of us that have, indeed, cast gold.

Note the variations here---none of which were desired. I did my level best to pour 10 ounce ingots. Two different times, in fact. Only by chance did I succeed, and only once in ten tries.

Harold

 

[MargueriteMingorance]

Here's a technical guide to making stuff out of fused silica:

http://handle.dtic.mil/100.2/AD766494

Basically, a fine slip is made of the milled silica (a claylike substance, in other words), which is molded however you want. This is allowed to harden, then is dried at around 200F to get all the water out. It is then sintered for 2-3 hours at 2100-2300F. Presto, you have a fused silica item.

You can get used kiln equipment on ebay for a few hundred dollars that will do the job. Finding silica slip is much harder, strangely. Even finding powdered silicon dioxide, or something similar, is difficult. I found chemical supply companies that charge $40/lb for it. Here is a place that sells pottery supplies, that sells a "silica 325 mesh", which I suspect is pretty pure silica powder. At $4.25 for a 5 lb bag (plus shipping) It is cheap enough to experiment with. Or, you could go with the 50 lb bag for $15 (more shipping).

http://store.clayscapespottery.com/products/silica-325-mesh-sil-co-sil

It's pretty sad, that I can find a lot of manufacturers in China that sell pure silica powder for $100 a ton. In the US, I find cosmetic companies and dietary supplements (!). It is a worrisome indication of the state of american industry.


BTW, last time I checked, Marguerite was a girl's name.

Hmm, yep, still no balls here. Whew.

 

[MargueriteMingorance]

Well, I'll edit out the insulting stuff, we don't need to see that again.

When one melts gold powder, the beginning weight and the ending weight is generally not the same.

Why?

Because the best washed gold powder still contains traces of something that isn't gold, and it tends to be reactive when heated. It is common for miniscule balls of gold to fly from a lot of gold powder being melted. It may be small, but small is all it takes.

What if you melted it, then atomized it with very pure water? Would you then be able to melt it again without that problem?

 

[qst42know]

This is more of a mental exercise than anything else.

If you did produce this silica mold it wouldn't cool in a fashion that would produce a desirable ingot.

Cooling from the top down the shrinkage of the ingot would suck such a cavernous pipe it would not be very attractive anyway wouldn't it?

 

[HAuCl4]

Here's a technical guide to making stuff out of fused silica:

http://handle.dtic.mil/100.2/AD766494

Basically, a fine slip is made of the milled silica (a claylike substance, in other words), which is molded however you want. This is allowed to harden, then is dried at around 200F to get all the water out. It is then sintered for 2-3 hours at 2100-2300F. Presto, you have a fused silica item.

You can get used kiln equipment on ebay for a few hundred dollars that will do the job. Finding silica slip is much harder, strangely. Even finding powdered silicon dioxide, or something similar, is difficult. I found chemical supply companies that charge $40/lb for it. Here is a place that sells pottery supplies, that sells a "silica 325 mesh", which I suspect is pretty pure silica powder. At $4.25 for a 5 lb bag (plus shipping) It is cheap enough to experiment with. Or, you could go with the 50 lb bag for $15 (more shipping).

http://store.clayscapespottery.com/products/silica-325-mesh-sil-co-sil

It's pretty sad, that I can find a lot of manufacturers in China that sell pure silica powder for $100 a ton. In the US, I find cosmetic companies and dietary supplements (!). It is a worrisome indication of the state of american industry.


BTW, last time I checked, Marguerite was a girl's name.

Hmm, yep, still no balls here. Whew.

How about picking some sand at the beach and milling it to size?. Or not milling it at all?.

Nice guide. Thanks!.

 

[T3sl4]

Possibly, a combination of silica fume and fine ground flint (flint is a form of silica which is commonly available very fine, e.g. 325 mesh) would do it. If it's still not sticky enough, you might experiment with bentonite (up to 5%), or perhaps fly ash (which may have a cementing effect and harden on its own).

Technical ceramics are typically made by pressing a powder with an organic binder.

Tim