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I am writing a setting for a role-playing game. In it an alchemist has finally found a way to create gold from lesser metals.

Like his former historical models the alchemist is so jubilant over his discovery that he seems to ignore that his "gold"...well, it really looks like gold and seems to be metallic, but some things like density, conductivity, malleability are absent. In short: It is not gold.

What I need is a substance which could have been mixed by an alchemist and is as gold-like as possible. May contain heavy metals or poisonous substances, the alchemist may be...a little affected.

The well-known pyrite is the soft-option, I do not find it particularly compelling as gold ersatz.

ADDITION:
Some additions to the proposed solutions

  • We are talking about a fantasy world with mediaeval technology level. All metals with a comparable density level are out of the question: tantalum, tungsten und uranium are out of the question, their necessary production conditions make them nearly impossible to produce. Platin metals which have an even higher density are even rarer than gold.

  • For the same reason radioactive decay or nano clusters are out of the question. It takes high technology levels to produce that result and the cost is astronomical.

  • Mixing gold in small parts does not work. In fact the experience with gold alloys shows that at least 75% volume per cent is necessary to get the given color.

Best solutions so far are brass alloys:

  • Nordic gold: Looks definitely like gold and is a metal.
    Composition: 89% copper, 5% aluminium, 5% zinc, and 1% tin.
    Problem: Aluminium cannot be produced because it needs high-powered electrolysis and rhyolite which is not available at this time.

  • Another very gold-like metallic alloy suggested by a friend is gold tombac or red arsenic tombac.
    Composition:
    Gold tombac >85% copper and zinc
    Red arsenic tombac: 98% copper and 2% arsenic

  • Lead iodine: Looks like gold powder.

For those who would like to know how characters can test gold: The metal workers and also the alchemists knew since ancient times that heating and melting gold was a very good method to test purity because normal alloys are separating. Another old method is the stroke test: Move both gold and the unknown metal over a stone and compare the results. Sounds crude, but is very, very good to detect fake gold. Density measurement is another option: Get real gold and a probe of the unknown metal with the same weight. Fill a container with water and drop the gold into it, so that the gold is completely submerged; mark the water level. If the unknown metal is gold, it will have the same level, if not, the water stands perceptibly higher.

ANOTHER CLARIFICATION: Many people have wondered or could not believe that alchemists which are capable to create new stuff are not able to admit that it is not gold. It sounds strange, but it is in fact true.

In "Alchemy: The Philosophers's Stone" from Allison Coudert it is explicitly mentioned in the chapter about transmutation that alchemists believed in several forms of gold. Geber said that gold is created from most subtle mercury substance and some pure, red, fixed sulphur substance which gives gold its color. Like the sulphur has different tones of red, so must have gold different shades of yellow. Chen Yin, a chinese alchemist, mentioned that the gold created from transmutation incarnates so many chemical ingredients that is superior than natural gold. Some of them drank actually tin (IV) sulfide as "liquid gold".

It is also mentioned that the alchemists stayed away from practical metal workers because they felt grandiose. Allowing to test their gold sounded like a sacrilege. There were also many, many stories about witnessed transmutations (Johann Konrad Richthausen, Wenzel Seiler und Nicolas Flamel. Yes, the guy from Harry Potter did exist in reality). So alchemists were easy prey to delusions of grandeur, their environment (mercury etc.) did not help exactly.

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    $\begingroup$ You might try asking on Chemistry what looks like gold?. $\endgroup$ – JDługosz Oct 9 '16 at 18:39
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    $\begingroup$ Since you want hard-science and it’s a real thing, asking there might give better info. The gold spectrum is rather distinct; nothing else looks like pure gold. $\endgroup$ – JDługosz Oct 9 '16 at 18:47
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    $\begingroup$ The trick to a good borderline question between worldbuilding and a hard science is in the acceptance criteria. "Looks like gold to a bunch of medieval alchemists" is hard to sell on Chemistry. "Looks like gold when viewed according to human trichromat vision" is easier. $\endgroup$ – Cort Ammon - Reinstate Monica Oct 9 '16 at 18:53
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    $\begingroup$ The issue is that, being as valuable as it was (and is), nobody is going to accept gold of unknown origin unless it is duly examinated by some expert (most likely a jeweller), only someone completely clueless (or self-deceptive) is going to fall for it. $\endgroup$ – SJuan76 Oct 9 '16 at 19:47
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    $\begingroup$ "The molecules tend to shift during the transmatter, uh, event....smells like cumin." $\endgroup$ – Kyle Strand Oct 9 '16 at 23:45

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Nordic gold might serve your purposes. It is an alloy which was developed for the Swedish 10 kronors coin.

89% copper, 5% aluminium, 5% zinc, and 1% tin

It clearly isn't gold but does look more like it than iron pyrite. It is clearly metallic while pyrite is too crystalline.

You can adjust the percentages here to realistically say that you found something that is even closer in color.

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    $\begingroup$ While it has not got so much attention yet, your answer besides brass is still the best one which is still attainable with lower technology. One of the problems with the answer is that aluminium cannot be produced before because it needs high-powered electrolysis. $\endgroup$ – Thorsten S. Oct 11 '16 at 19:43
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    $\begingroup$ Your figure does need to discover something new in order to explain why he doesn't recognize it. Any self respecting alchemistic should at least recognize brass right? While pure aluminium is impossible for him it is believable he could produce an impurity of aluminium (though 5% is pushing it). I am more concerned that he didn't label the Nordic gold as simply a type of brass. $\endgroup$ – kaine Oct 11 '16 at 20:35
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    $\begingroup$ @ThorstenS., aluminum can be reduced from the ore by reacting it with metallic potassium or sodium, and sodium can be produced through a non-electrolytic process. $\endgroup$ – Mark Oct 11 '16 at 21:06
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    $\begingroup$ @dsollen There are only few elements which are found in native form, aluminium is exceedingly rare and would have (if discovered) been extremely expensive, much more than gold. $\endgroup$ – Thorsten S. Oct 11 '16 at 21:09
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    $\begingroup$ @ThorstenS., heat sodium carbonate (your alchemist will probably know it by then name of "soda ash") with powdered coal to 1100 °C to produce metallic sodium and carbon monoxide. $\endgroup$ – Mark Oct 11 '16 at 21:14
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What I need is a substance which could have been mixed by an alchemist and is as gold-like as possible.

How about gold itself? Bear with me, here:

While there are a lot of things that look kinda like gold, nothing quite looks as good as the real thing (as pointed out in Cort Ammon's most excellent answer). So, why not use the real thing and have your alchemist accidentally stumble across gold plating, which of course is simply the process whereby relatively very small amounts of gold are deposited onto the surface of an object made of a different substance, ideally with a similar density.

How could he do this with typical medieval kit (i.e., without readily available electricity)? According to this paper in JOM, researchers were able to replicate the results of pre-Columbian smiths in Northern Peru using an electrochemical reaction by dissolving gold into an aqueous solution of NaCl + KNO3 + KAl(SO4)2 · 12H2O, achieving gold plating onto copper. The chemicals would have been readily available minerals to a medieval (al)chemist, consisting of ordinary salt, saltpeter, and potassium alum. In fact, they are all naturally-occurring.

Alchemists I've seen in fiction tend to use hilariously complex apparatus, have spent many years sniffing mercury vapours, and have weathered many, many failed attempts in search of that elusive ? ⟶ Au formula. It wouldn't be hard to imagine an alchemist cracking open a dusty tome that suggests they need small amounts of the target element as a catalyst to start the reaction. Trembling in anticipation, they add in a pinch of gold dust into the above solution, and soon their entire hunk of metal appears to be pure gold!

Of course the gold plating would be discovered it as soon as anyone scratched it hard enough, but I think that's the intent, right? Something that is initially very convincing, but doesn't survive scrutiny.

You specified "it's not gold", and while this would have a very small amount of gold in it, it's still 99.9% "not gold", which I hope satisfies the spirit of your criteria.

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    $\begingroup$ This. You could even have the "secret formula" to—at it's core—be simply boiling an element in gold. Maybe the addled scientist was never smart enough to cut it open and check the gold was solid throughout. $\endgroup$ – Mirror318 Oct 11 '16 at 1:15
  • $\begingroup$ I would add that my vauge memory of the actual 'lore' of alchemy would say that it's entirely in keeping with some academical views that gold could be required to produce more gold. The way they treated the elements was different and they had some rather mythological aspects mixed into their science. I believe one of them was the idea of like elements being linked in a way that would lead it's self to gold making more gold. Though i don't swear to know the lore well enough to say for certain that I'm not confusing two different concepts. $\endgroup$ – dsollen Oct 11 '16 at 20:48
  • $\begingroup$ Thanks @dsollen . If you happen to find a source for that, I'd love to add it to my answer (or go ahead and edit it in yourself, if you feel like it). Bonus if it's an actual historical reference in keeping with hard-science. $\endgroup$ – type_outcast Oct 11 '16 at 21:31
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When I was younger, as part of a science summer camp we made pennies turn to gold. I don't know what the bar will be for "looks like gold," but it seemed like a good starting point for me. The process was:

  • Start with copper pennies (not the zinc filled ones we have today)
  • Zinc plate them (which makes them look silver)
  • Heat them over a Bunsen burner until the zinc and copper meld into brass (which looks "gold" to a bunch of kids)

Brass penny Gold Kuggerand

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    $\begingroup$ @JDługosz To the best of my knowledge, the penny is British. $\endgroup$ – Cort Ammon - Reinstate Monica Oct 10 '16 at 3:50
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    $\begingroup$ The penny is definitely British - it says "GRA.BRITT." on the heads side and has Britannia on the tails, complete with a union flag on her shield. $\endgroup$ – Nathaniel Oct 10 '16 at 3:56
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    $\begingroup$ An alchemist that is unable to recognize bronze seems a bit of a stretch though. Especially if he made it by mixing copper with other metals. $\endgroup$ – Taemyr Oct 10 '16 at 8:36
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    $\begingroup$ Zinc & Copper is brass, not bronze (Copper & Tin), but it does have that goldish kind of look and would seem to satisfy the question. $\endgroup$ – colmde Oct 10 '16 at 14:56
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    $\begingroup$ @JDługosz Sheer luck? I can only assume that there is a "correct" lighting to photograph coins as far as numismatists are concerned, because you're right -- its almost creepy how similar the lighting is! $\endgroup$ – Cort Ammon - Reinstate Monica Oct 10 '16 at 15:27
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A way to do this would be to allow "success". Our friend has actually found a way to fission Lead into Gold. In particular Lead-206 into Gold-198, via the chain -alpha-> Mercury-202 -alpha-> Platinum-198 -beta-minus->, the latter isotope having a half life of 3 days. That's a long time for something to be chemically gold before decaying to (mostly) Mercury. Alternatively, Gold-195 has a half-life of 186 days, but mostly decays to Platinum, which may not count as "failure".

Edit:

Useful tables of decay products and sources:

Lead-204 is naturally occurring in the absence of uranium, actinium, and thorium decay chains. Lead-206, -207, and -208 are products of those chains, respectively. Those four variants of lead are "stable".

  • Lead-204 -alpha-> Mercury-200 with half-life >10^20 years
  • Lead-206 -alpha-> Mercury-202 with half-life >10^21 years
  • Lead-207 -alpha-> Mercury-203 with half-life >10^21 years
  • Lead-208 -alpha-> Mercury-204 with half-life >10^21 years

Thallium-203 and -205 are stable.

  • Thallium-203 -alpha-> Gold-199 with unknown half-life

Mercury-196, -198, -199, -200, -201, -202, -204 are stable.

  • Mercury-198 -alpha-> Platinum-194 with unknown half-life
  • Mercury-200 -alpha-> Platinum-196 with unknown half-life
  • Mercury-202 -alpha-> Platinum-198 with unknown half-life
  • Mercury-203 -beta-minus-> Thallium-203 with 47 day half-life
  • Mercury-204 -alpha-> Platinum-200 or -double-beta-minus-> Lead-204 with unknown branching fraction and unknown half-life.

Platinum-192, -194, -195, -196, -198 are stable. All of these decay via alpha emission to various Osmium isotopes.

  • Platinum-198 may also twice beta-minus to Mercury-198 with a half-life >10^14 years.

Gold-198 is the intermediate in the above Platinum-198 twice beta-minus chain. It has a half-life of 2.7 days.

  • Gold-199 -beta-minus-> Mercury-198 with half-life 3 days

The above give decay chains from Lead-206 and Lead-207 to Gold-199. The only hard science problem here is encouraging these various alpha emissions so our friend's reaction doesn't take many, many times longer than the current age of the universe. I recommend Fly Amanita. (Not really.)

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    $\begingroup$ What you are describing isn't fission. Hg-202 -alpha would give Pt-198. And I can't find decay data for Pt-200. I also don't see how you're getting to Au-195. $\endgroup$ – Loren Pechtel Oct 10 '16 at 19:57
  • $\begingroup$ Reminds me of this story. $\endgroup$ – JDługosz Oct 10 '16 at 21:40
  • $\begingroup$ @LorenPechtel : Yeah. Thinko'ed the Platinum weight; fixed now. (Platinum-198 is stable, but clearly our friend has a trick. I assume it involves Ancestor Moth Wing.) Platinum-200 (half-life 12.5 hours) decays by beta-minus to Gold-200. I hadn't bothered with working out a Gold-195 decay chain as it would be outstandingly unlikely. $\endgroup$ – Eric Towers Oct 11 '16 at 15:34
  • $\begingroup$ I'm sure he would be happy with discovering fission for a few nanoseconds before his body and home were destroyed by the resulting explosion (well, to lazy to do the math, it either releases a massive amount of energy or takes a massive amount). Plus the radiation... $\endgroup$ – dsollen Oct 11 '16 at 20:51
  • $\begingroup$ @dsollen : If Fleischmann & Pons ideas on getting nucleii much closer together (to lower the activation energy of nuclear reactions) actually worked, the activation energy, half-life, gamma radiation, neutron radiation, and waste heat would be substantially lower. Maybe one-tenth!!! ;-) $\endgroup$ – Eric Towers Oct 11 '16 at 23:12
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You can try a reaction between potassium iodide and lead nitrate; the resulting lead iodine will be visually somewhat similar to gold powder. It involves the usual trope of turning lead into gold, and as it is a lead salt, it is poisonous. It is also not a metal, so, unlike brass, nordic gold, or other copper alloys, it has no metallic properties such as malleability, electrical/thermical conductivity, etc.

If your alchemist is minimally competent, he will know this isn't actually gold, but he may deceive himself that it presents a step in the right direction... or he may (try to) deceive others, as long as he doesn't allow them to examine it more closely.

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Based on an article scientists have discovered that a specially crafter silver structure can look, act and behave exactly like a gold-based structure.

The entire negatively charged, silver-based complex ion has the chemical formula [Ag25(SPhMe2)18]-. Although a few other silver nanoclusters have been synthesized in recent years, this is the first silver nanocluster that has a matching analogue in gold: [Au25(SPhMe2)18]- has previously been reported. Besides both nanoclusters having 25 metal atoms and 18 ligands, they also both have all of their atoms and electrons arranged in almost exactly the same way. In their study, the researchers performed tests demonstrating that the silver and gold nanoclusters have very similar optical properties. Typically, silver nanoclusters are brown or red in color, but this one looks just like gold because it emits light at almost the same wavelength (around 675 nm) as gold. The golden color can be explained by the fact that both nanoclusters have virtually identical crystal structures.

Note that this doesn't say that the molecule based on silver atoms behaves like gold, it just says that the structure behaves similarly to a particular gold molecule structure, but which includes some of the properties gold has.

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    $\begingroup$ This is a "link-only answer", and that is generally not accepted here. $\endgroup$ – Hohmannfan Oct 10 '16 at 16:14
  • $\begingroup$ I think that the main problem here is the behaves exactly like gold portion - This may not fit within the stated question. $\endgroup$ – Thebluefish Oct 10 '16 at 16:49
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What about Brass?

I think it actually was discovered by alchemists that thought they finally found gold.

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  • $\begingroup$ stereotypical medieval alchemists came about much later than the discovery of brass...and this is was Cort already said in more detail. $\endgroup$ – kaine Oct 10 '16 at 18:22
  • $\begingroup$ Well back then Cort wrote Bronze and changed that later to Brass $\endgroup$ – Jannis Hell Oct 11 '16 at 7:59
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I see you have an accepted answer already, but I think you'll still find this useful. I did an experiment in my high school chemistry class where we turned pennies into "gold" using a few chemicals. These chemicals are readily available now, I don't know how easy they would be to extract and purify in those times. That'd require further research. But the basic experiment (full steps at this link) was simple. You had to get zinc lumps, water, and zinc sulfate solution mixed in a beaker and bring to a boil. This will cause the pennies to turn from a copper color to silver. Remove the pennies from that solution, and heat them (either directly in a flame or just placed on a hot enough surface), and they'll form a brass alloy coloring. I know this can stay for years, as I did this experiment in 2008, and I cleaned out my room last week (October 2016) and that penny is still gold. Keep in mind that modern pennies are a very thin layer of copper over zinc.

So this, along with a little bit of speculative history, gives me an idea of how you might be able to fool many people into thinking you have gold. You need your lead of course, then the zinc and zinc sulfate as mentioned in the last paragraph, a little bit of copper, and a Baghdad battery.

I did a bit of googling and it is kind of conflicting, but it might be possible to electroplate copper onto lead. A chemist would need to tell you more about that, as I am not one. But on the Baghdad Battery link above, the tv show Mythbusters made a few replicas and proved it was possible to electroplate some metals in copper, at least in small quantities. And the batteries themselves are ancient, so they can be constructed for sure. If you can get the copper onto the surface of your lead object using the batteries, and then run it through the experiment in the first paragraph, you could conceivably form a brass shell around it convincing enough to fool most people into thinking it was gold. With the added benefit of the density of lead being kinda close to the density of gold (at least close enough to fool someone who hasn't spent much time around gold), since it would still be mostly lead anyway, you could make "gold" coins from lead and fool all but the most sensitive measurements available at that time.

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  • $\begingroup$ Alas, too late: Cort had already pointed out the method for brass coating of pennies. And the density of lead (11,3 g/cm^3) is nowhere near the density of gold (19,3 g/cm^3). $\endgroup$ – Thorsten S. Oct 11 '16 at 20:49
  • $\begingroup$ Who are you trying to fool? Other alchemists/gold smiths, or anyone else? The education to realize what was going on wouldn't be widely available for hundreds of years (you could argue that it still isn't, at least in some places). You can certainly pass it off as gold in the majority of situations, even if it is provably not gold to anyone who knows what they're doing. Its not like paper money today where you can hold it up to a light and catch fakes much more easily. Plus, Cort's answer only covers a small part of the process I have described here. Mine takes you all the way through. $\endgroup$ – Cody Oct 11 '16 at 20:55
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How much are we confined to medieval chemistry? If not - gold, but an unstable isotope would have quite interesting properties storywise; turning into lesser metals over time, emitting deadly radiation ... Perhaps you could even make a cursed ring out of it, possessing the ring-bearer slowly?

Getting unstable gold in the first place is a bit hard, but if you can get high-energy particles into your story, it would be quite possible making from lesser metals.

See Wikipedia - Synthesis of precious metals

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Note that ever since ancient Greece, measuring density by combined weight and volume displaced has been well known ("eureka!"), as has plating/leaf application, so producing something that survives even basic testing and still seems to be gold is more difficult than it might seem. There are also some other tests.

Fake alchemists often used a small amount of real gold to "prove" they were on the right track and just needed more time (and investment) to scale up the process.

I actually like the idea of someone scamming a legitimate alchemist, though it would have to be one who didn't know the usual cheats.

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  • $\begingroup$ "Fake" alchemists? As opposed to what? $\endgroup$ – Malvolio Oct 11 '16 at 7:26
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    $\begingroup$ Many, probably most, alchemists were honest experimenters. Some of their goals were mistaken in retrospect, but they believed in what they were doing and did it well within those limits. A relatively small number resorted to faking results to get financial backing, or were pure charlatans. Credit where credit is due, please. $\endgroup$ – keshlam Oct 11 '16 at 11:40
  • $\begingroup$ This world might not have had an ancient Greece. I'm sure in an alternative history we would reach medieval technology with a different set of background discoveries. $\endgroup$ – Rodney Oct 12 '16 at 14:24
  • $\begingroup$ Certainly possible, @Rodney, but I don't think you get very far in alchemy without some ability to test what's in front of you... And without some good test, who cares if it's real gold or not? $\endgroup$ – keshlam Oct 12 '16 at 15:34

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