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Consider a pre-scientific human civilisation. They have some alchemy nonsense, and are used to experimenting with boiling, filtration, etc, but do not have an atomic theory, or a mechanistic view of chemistry.

What is their plausible path to growing a pure crystal?

(EDIT: I'm looking for a large, regular crystal of a somewhat hard/durable substance. I regret mentioning salt/sugar in my example below because these form rather brittle crystals.)

The crystal can be any common substance (for example, NaCl) but they need a process to purify it. Suppose salt and sugar are mixed together, for example -- both are water soluble, so it is not obvious (even to me) how to separate these components.

The resulting purification does not have to be perfect, just "pretty good", but it has to be achievable with commonly available or easily manufactured materials.

What sequence of discoveries could result in this?

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    $\begingroup$ are you asking specifically about separating sodium chloride from sugar, or in general? Because the answer strongly depends on the involved species $\endgroup$
    – L.Dutch
    Mar 5 at 10:03
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    $\begingroup$ Proto-scientists like Cesalpino were growing pure crystals by at least the 16th century in real life, long before anything even vaguely resembling a modern scientific understanding of chemistry had precipitated out of alchemy nonsense. Does that count, or is that too recent? (Also, ice is a crystal, so if that counts people have been manufacturing it technologically since antiquity.) $\endgroup$
    – g s
    Mar 5 at 17:08
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    $\begingroup$ Sugar is very very soluble, salt is much less so. Fractional crystallization is what's needed and a crude version of it would not be that difficult to achieve and would probably occur naturally to a limited extent. Think of small salt water pools cut off at high tide. If you look closely you will often see salt crystals around the edge of the pool as the water evaporates. $\endgroup$
    – Slarty
    Mar 5 at 18:13
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    $\begingroup$ What does 'scientific' mean to you, please? I see purifying substances and growing crystals as simple fact, which science follows. I see 'scientific' as trying to explain how and why. Don't you? If a pre-scientific civilisation has 'alchemy nonsense' why not explain or drop that? If a civilisation is experimenting with boiling, filtration, etc, how is that 'pre-scientific'? Who told you science demanded an atomic theory, or a mechanistic view of chemistry? Are they not treading the same route as our revered ancestors? Why would their path to growing crystals differ from yours? $\endgroup$ Mar 5 at 21:55
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    $\begingroup$ Crystallisation is a very effective way of purifying many substances. $\endgroup$
    – Karl
    Mar 5 at 22:03

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They can do it surprisingly easily. All they would need is a series of containers in which the solution can evaporate. Each substance would have its own saturation level, beyond which it can no longer stay dissolved, and the substance with the lowest saturation level will precipitate out of the solution first, at which point they transfer the remaining solution to a next container. If they're after the substance which happened to precipitate, good! They're done. If not, they repeat the process until it is their desired substance's turn to precipitate out of whatever remains of the solution. If they want the crystal to be nice and regular, no problem: just dissolve the substance in a suitable solvent on its own (and how convenient that they just got rid of most impurities, which have either precipitated out earlier or remain in the discarded solution) and let it precipitate out again in a more controlled environment.

They do not need any elaborate theories to do it either. The ancient Phoenicians used exactly this process to extract salt out of seawater, using nothing more than a series of shallow ponds. Sea salt is mostly NaCl, but also contains smaller amounts of other salts. I'm not sure if they wanted to get rid of them or to have more of them, but either way they controlled the mix of salts in the final product just to make it taste better.

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    $\begingroup$ Not so easily after all. When a solution becomes supersaturated for one of the solutes, the excess solute precipitates until its concentration in the solution comes down far enough. That still leaves some of that solute in solution. Also, just because substances have different solubilities does not mean that they can't or won't crystallize from the same solution. Furthermore, sometimes different chemical compounds co-dissolved in the same solution will co-crystalize as crystals that contain both (as opposed to separate crystals of each). $\endgroup$ Mar 5 at 20:04
  • $\begingroup$ @JohnBollinger, thank you. Of course; that's a cheap and cheerful 'good enough' method, just as OP asked... ;) $\endgroup$
    – ihaveideas
    Mar 6 at 1:27
  • $\begingroup$ @JohnBollinger That is why my answer uses alcohol instead of water. The process is the same, but water is not always the best solvent for this process. $\endgroup$
    – Nosajimiki
    Mar 6 at 19:39
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To answer the specific example about salt and sugar

Separating salt and sugar is an easy one that any medieval alchemist should know how to do. Both are similarly water soluble, yes, but sugar is much more soluble in alcohol than salt. So, if you have a pile of salt and sugar, and need to seperate them, dissolve the mixture in alcohol. The salt will sediment out of the solution while the sugar stays dissolved. Let the alcohol evaporate until it no longer tastes salty, then pour it off leaving you with relatively pure salt crystals and then let it continue to evaporate until it leaves behind relatively pure sugar crystals.

If the crystals are not pure enough, repeat the process until it is. Refining is all about repeating a process until a desired level of purity is achieved.

Worldbuilding with Alchemists vs Chemists

I think the real question here has more to do with understanding the limits of what a believable alchemist can and can not do. For starters, just because alchemists don't know the periodic table does not mean they are not experts in thier fields. Common stuff like salt, water, sugar, alcohol, etc would all be things an alchemist would have a lot of knowledge about; so, you should assume that an alchemist can do most of the same things with these that a modern chemist could do given basic lab equipment.

What they can not do is make complex predictions about things that were not already common; so, if they wanted to invent or learn to refine a new material, it was done through rigorous experimentation, not through taking materials based on known chemical compositions and recombining them through stoichiometry. This made refining processes that require many intermediate steps much harder to stumble upon by accident.

If you want to think about how alchemists really work differently than chemists, think about Thomas Edison. He experimented with over 3000 random materials before settling on Tungsten filaments for his lightbulbs. Nikola Tesla complained that if he used chemistry, he could have thrown out 90% of those materials based on known chemical properties, and discovered Tungsten filaments much faster. Both inventors were able to come to the same conclusion that Tungsten was the best solution, but it took Edison a lot more experimentation to get there.

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  1. The word "substance" used to have a very different meaning in pre-scientific times. It was only when chemistry began to emerge as a science that chemists hijacked the ancient word substance, which for a long time had meant the "essence", the "inner being" of a thing (as in, for example, transubstantiation), or, alternatively, "wealth" (as in, for example, "a man of substance").

    The Latin word substantia was coined by the philosopher L. Annaeus Seneca "the Younger" in the 1st century of the common era in order to translate the Greek word hypostasis; the basic meaning of both words is transparently "that which underlies something". Many such copies, or calques as linguists say, of Greek words were introduced by Roman philosphers striving to overcome what they perceived as the poverty of their native language.

  2. In light of their lack of a modern definition of a chemical substance, they also had a rather fuzzy understanding of what "purifying" it meant.

  3. These being said, there do exist a number of simple purification techniques which have been used for a very long time. Wikipedia has a list of purification methods which can be mined profitably. Generally, purification methods can be either physical (such as fractional distillation or evaporation), which exploit differences between the physical properties of different substances, or chemical, exploiting the different chemical properties of different substances.

    • Where there is a will, there is a way. The ancients were very good at purifying gold and silver, because there was a very strong incentive for them to be very good at purifying gold and silver. Typically, their purification technology achieved about 99% purity of gold or silver.
  4. Given the specific problem, there are multiple ways to extract the salt:

    • Evaporate the water, then calcinate (= burn) the residue. Sugar burns, salt does not burn.

    • Evaporate the water, then attempt to dissolve the residue in alcohol. Sugar is very much more soluble in alchohol that salt is, so that most salt will sink to the bottom.

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Crystallisation is a technique for separation and purification. If you leave your solution of salt and sugar to evaporate, you will get sugar and salt crystals which you can distinguish by sight and separate by shape. The crystals may have some contamination; but if you dissolve the salt and the sugar in separate amounts of water and repeat the crystallisation, the result will be purer.

It is hard to say what discoveries are possible. It is tempting to speculate that crystal structures imply atomic structures. However, an alchemist is not going to know that Sodium Chloride is made from alternate ions of a light silvery metal and a green gas. Dalton's atomic theory was based on the fact that chemicals reacted to form other chemicals in the same mass ratios.

Pasteur noticed that sodium ammonium paratartrate in solution was not optically active but crystallised into two forms that were mirror images of each other. It is a major leap to assume your alchemist would look at the exact shape of the crystals, or observe them through some polarising medium such as Iceland Spar. However, if they did, they might notice that some chemicals had left- and right-hand forms, but the same chemicals obtained from a living source only had one form. This was used in a short story from 1930 by Dorothy L Sayers 'The Documents in the Case'.

IMHO, crystals were known to the ancients. They gave tantalising clues to some form of geometric regularity underlying the nature we see. But, apart for crystallisation being used as a separation and purification technique, we did not learn much from crystals until relatively modern times.

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The problem is not likely the purification of a solution, it's most likely the temperature required for sought solution to be liquid, and forces involved to maintain the solution's concentration and/or crystal growth. Most today's crystals are grown in autoclaves or stronger machinery, with solutions/melted substance being heated to several hundred degrees Celsius, and also oftentimes pressurized. (I'm not speaking diamonds, they require volcanic heat and impressive pressure just to transition to phase when diamonds can form). With a substance like NaCl, growing pure crystals could be done by pre-scientific people with relative ease, as this crystal can be formed under normal conditions (and currently there are instructions to grow a NaCl crystal at one's kitchen), with a substance that is solid at normal conditions, growing a crystal out of it would require a lot of controlled heating and possibly also applying pressure, while alchemists knew about heating to change "substances", they didn't know what causes them to change, thus producing a pure artificial crystal of such a thing before advent of science is borderline impossible.

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There is a really neat process for purifying crystals where all you really have to is just be able to melt stuff...

It's called "Zone Refining" (or more generally, "Freeze Distillation"), and it's a process where you successively purify a substance heating a small section to melting and then slowly moving the "melt zone" along the sample over and over again.

The reason this works so well is because "crystals beget more crystals", that is to say, crystalline things want to form nice ordered crystals (funny that 😉) and this has the side effect of pushing a lot of the impurities out of the way in the process... So, by moving the melt zone along your sample over and over again, you get increasingly pure crystals at one end and end up concentrating the impurities at the other end. Lather, rinse, repeat until the desired level of purity has been achieved.

Although, there are some caveats to keep in mind if you're going to be doing this in ancient times:

  • If it reacts with air, forget it (you're just scooping water with a sieve at that point)
  • If it melts above ~1300'C, forget it (melting steel is hard enough and you want more heat?)
  • if it's sensitive to cooling rates, forget it (some things will just crack and shatter like glass)
  • if it's highly corrosive to the pot, forget it (you don't want your sample to bond with and become one with the container...)

None of these are 100% deal-breakers, but they will make your life substantially harder without access to modern materials and process controls.

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Repetition purifies in a closed system, even in medieval times. Make a closed kiln, were the crystall substance boils. Add a bowl of substance nearby that reacts and binds a polutant that gets airborne. Then heat and repeat.

Over time, a enormously wasterfull process, might produce something the industry can not produce.

This is the same princip modern precision manufacturing was build upon. You can not build it, but you can measure it, so you build with tools that can not build it, a tool that can build it. Then you can no longer measure it and need to build more precise tools.

The paradox nature of the improvement already communicates the waste involved. But finally your medieval society, will have a waver, of pure crystal meth..

Post Scriptum: One example for this "start of process" are lathes. https://en.wikipedia.org/wiki/Lathe#History There are increddible old artifacts of lathes.. and every lathe gave birth to more precise lathes, spawning again more precise instruments.

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(EDIT: I'm looking for a large, regular crystal of a somewhat hard/durable substance. I regret mentioning salt/sugar in my example below because these form rather brittle crystals.)

Frame challenge: they used glass instead.

They might be able to grow some small crystals from melts, but growing large crystals of any durable material requires good temperature control at high temperatures for long periods of time, which is going to be beyond their capability. Their only real means of producing large crystals is by crystallization from water solution, and while it is easy to produce some large crystals this way, they are water soluble and sensitive to humidity and temperature. For example, it is almost trivial to produce finger-sized crystals of sodium sulfate decahydrate, but they will redissolve in their own water of hydration if warmed a little (below blood temperature), and if left exposed will lose that water to evaporation and turn into powder.

As for general mechanical durability, there's little that they could produce that would have properties competitive with glass. People were making glass beads five to six thousand years ago, and tiles, vessels, and other objects by the late bronze age.

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This is not a very hard question actually. Pre historic civilizations used copper, iron, steel, gold, silver, with different rates of purification. Actually it was a common practice to devaluate money simply by decreasing the amount of precious materials they include.

They could simply get the pure (or very near pure) substance by smithing. It might be a bit harder to shape the crystal. It is also a smithing process.

I'd say what smiths have done in dark ages are how you get a crystal in pre science society.

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    $\begingroup$ getting a crystal has little to nothing to do with smithing. A crystal is not a matter of shape, it's a matter of ordered atoms in a lattice. $\endgroup$
    – L.Dutch
    Mar 5 at 10:43
  • $\begingroup$ To clarify on LDutch's point, yes all solid metals are crystalline solids, and smithing is a method of purifying a metal... but I think there is some confusion in the statement "It might be a bit harder to shape the crystal." since metals are more accurately a solid made of tiny crystals rather than a single crystal. Though in theory, if you were to very slowly cool a molten purified metal, you could get large metallic crystals. Though some metals like Beryllium are pretty easy to get macroscopic crystals out of. $\endgroup$
    – Nosajimiki
    Mar 5 at 16:07
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Fractional freezing can replace distillation in some cases.

enter image description here

Strong ice beer (the percentage of alcohol has been increased till unusually high 12 % by fractional freezing), image credit.

Let's suppose we have a close to saturated solution of some substance contaminated with other substances that are more soluble. Cooling down such a solution (like freezing outside in winter) will force our substance to fall into the sediment while more soluble contaminants will remain in the solution. We can now add just water, warm up to get our substance solved again and repeat multiple times.

Discarding less soluble contaminants is even easier. Cool down / freeze and they fill fall into sediment while more soluble substance of interest will remain in the solution.

Surely this may not work well with every substance, every solvent and every contaminant, but if it can be done, it can be done in a very simple way, more or less with just a bucket and some source of alternating temperatures (heated room, fireplace, hot springs, day vs night, etc).

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Sugar burns at 350 degrees F. Salt burns at 1474 F.

Just heat the solution to a temperature between 350F and 1474F (say like 500F) and all the water and sugar will disappear leaving only the salt.

Also keep in mind that you have to purify the water. But that can just be done by boiling it and capturing the steam (distilling the water).

For your primitive society to create this process...

  • They would need to notice that steam condenses, and they can collect the purified water.
  • They would need to recognize that salt and sugar burn at different temperatures
  • They would have to work out a way to burn something at roughly the right temperature (in this case within +/- a few hundred degrees).
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  • $\begingroup$ I am not sure why this is getting downvotes. In my college chemistry class, we did a lab where we separated two substances in a crucible by burning one of them off. It's definitely a viable process depending on the specific materials you are trying to separate. $\endgroup$
    – user4574
    Mar 10 at 0:23

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