How can I roughly measure a fraction of a gram?

My son got a crystal making kit (from a relative) and it lists seed crystal weights of 0.5g for a small crystal solution and 0.75 g for a large crystal solution. The trouble is I (his father) don't have a scale that can measure a fraction of gram, so I guessed how much.


Can I roughly get the weight that a fraction of a gram without buying a scale just for this crystal making kit?




You ask:



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> Can I roughly get the weight that a fraction of a gram without buying a scale just for this crystal making kit?
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Absolutely.


You're in Japan. You have access to [Japanese coins](https://en.wikipedia.org/wiki/Japanese_yen#Coins) which conveniently are just what you need for this project. Because they are currency, you can also be assured that they weigh exactly as advertised.


* You need one each of 5-yen, 10-yen, and 50-yen coins
* For the scale, make something like [this balance](https://www.youtube.com/watch?v=FRtbekqsuZ8), and note that you might want to scale it down given the small amounts you're working with
* To get 0.5 g, add enough crystal to the cup with a 50-yen coin in it such that it's balanced with the one with a 10-yen coin in it
* To get 0.75 g, add enough crystal to the cup with a 5-yen coin in it such that it's balanced with the one with a 10-yen coin in it


I suggest you tape the coins to the weighing cups so you don't have to get the powder all over them. You might tape the coins on the underside of the cups and be sure to use a similar size section of tape so the weight of it is more or less equal.


 12

I think the mass of the seed is not critical. But you can create your own scale - or even better, involve the son in its creation.


You can create a 2-shoulder scale, with 2 stuff holders hanging at equal distances from the central holding.


Or, you can create a 1-shoulder scale, with the stuff holder hanging on some elastic string or wire.


For a scale weight for the former, or as the calibration of the latter, you can use the fact the typical water drop has volume $\pu{0.03 mL}$, i.e. $\pu{30 mg}$. You can check the actual value by counting drops filling a known small volume.


You can find a web tips or precedures to make homemade scales like in this [Kid science](https://www.thoughtco.com/kid-science-make-a-balance-scale-2086574) article.


 8

If you know the density of the crystal (usually a salt), you can estimate the mass of a crystal based on its volume, i.e. by measuring the lengths of its sides.


For example, table salt (sodium chloride) has a density of $\rho=2.16\ \mathrm{g\ cm^{-3}}$.  

Thus a cube of $10\ \mathrm{mm}\times10\ \mathrm{mm}\times10\ \mathrm{mm}$ has a mass of about $2.2\ \mathrm g$.  

And a cube of $5\ \mathrm{mm}\times5\ \mathrm{mm}\times5\ \mathrm{mm}$ has a mass of about $0.3\ \mathrm g$.


The precision of this estimate should be enough to select a suitable seed crystal.


 7

Very roughly, you could take a hint from the old British Imperial measurements. The smallest unit of weight is the *grain*, which before standardisation was the weight of a single grain of wheat, which was then standardized as something which translates roughly to 64mg. (Although with plant breeding since then, I would not be surprised if modern wheat grains are much heavier! )


I suspect rice grains are lighter than wheat grains.


You could weigh some grains of a raw cereal (or peppercorns, mustard seeds ...) against a coin of known weight, count them, and divide, to get a somewhat more accurate idea of the weight of each.


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Use a balance with uneven arms like in a factor of 1:10. Test it with known weights to see if the factor is correct or recalculate to the exact factor and use it to measure with the lowest weight on the longest arm.


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