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The Supposed Use of Grains, and Seeds, in Historical Metrology
.....I recently came across the Wikipedia page on carats (extracts repeated at the end), via a Yahoo discussion group, and thought to comment on it fully here – since confusion on the nature of metrological grains, carats, rattis etc seems to be very widespread
The Wikipedia article is at:                http://tinyurl.com/yksxpzf

.....Wikipedia rejects the theory that. “carob seeds are naturally more uniform in weight” (than other seeds) and cites research reported in “New Scientist”.  I do not challenge that claim.
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.....However, Wikipedia then suggests to the reader a dubious second theory, that: “Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight”.

.....I would argue that this is at root a popular myth.

.....I agree that etymologically “carat” very probably derives from the name of the carob seed, but that in itself does not mean that at any point in recorded history knowledgeable people actually used randomly selected carob seeds on “precision” scales.  Consider for a moment the “foot” as used in the USA.  Etymologically it surely derives from the human foot – but its length has nothing to do with the actual feet of US citizens.  Likewise, the British “stone” has even less to do with the size of the stones one might come across in Britain.

.....We have evidence that suggests the Babylonians began to standardise their weights, as replicas of an official stone standard, by at least 2,000 BC. This would accord with common sense, and I believe is the key point to notice in connection with such matters.   In as far as we can tell, this sensible practice has been widely followed at most places and times ever since.  So whilst randomly selected carob seeds may once have serviced as weights, sensible individuals stopped using them to determine their precise weight standard more than 4,000 years ago.

.....So – if carat weights do not generally come from randomly selected carob seeds – where do they come from?  Wikipedia discusses three different sorts of carats – as follows
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a)  the Roman carat (siliqua).  Now if we accept Wikapedia’s own figures then the Roman pound was about 327.45g, (represented by an official example held in the temple of Juno Moneta) and it comprised 72 x 24 carats – which thus weighed 0.189g.  Thus in the Roman scheme of things, the weight “carat” is just the name of an arithmetical fraction of the official standard pound – 1/1728th to be exact

b)  the “Syrian” carat.  This weight derives from the weight of the gold dinar, which most likely derives directly from the weight of Alexander’s drachm – c.  4.25g  (although Grierson failed to realise this).     The Syrian carat is exactly 1/20th of this dinar   -   thus c. 0.212g.  So again the carat is just the name of an arithmetical fraction, this time of a standard gold coin.
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c)  the “Egyptian” carat.  Wikipedia claims to follow Grierson and fixes this as c. 0.196g.   In fact, Grierson fixed this carat at c. 0.1968g, based upon a study by Miles in 1948 of glass weights.   In 1964 Miles published a study based upon a much bigger sample of weights, and revised the estimate to c. 0.1947g

.....Wikipedia follows Grierson and Miles in suggesting that the weights were probably calibrated to local Egyptian carob seeds.  But Miles and Wikipedia fail to take into account a long existing weight system of Egypt, which very closely resembles, and probably gave rise to, the English troy system.  In its original form this seems to have comprised a ("16 oz") pound of about 500g (the Ratl Kabir or great pound) divided into 16 oz of 10 dirhems of 64 (“wheat”) grains.  .  Today we use it in  it in England as 12 ounces of 20 pennies of 24 (“barley”) grains.  (where 4 “wheat grains” are exactly equal to 3 “barley grains”)  According to modern standards, this yields a (“barley”) grain of 0.064799g.  Three such grains therefore equal 0.194g.  Thus yet again this Egyptian carat looks a lot like a simple fraction of a standard Mina.  In Islamic Egypt this would have been expressed as 1/16th of a dirhem, which in turn went 120 to the 12oz pound.  In England as 1/8th of a penny, which went 240 to the same pound

.....Thus it seems that in every case the weight of "the carat" can be readily explained as being the result of a simple method of arithmetically dividing a standard large weight into smaller weights (of about 0.2g).  Equivalent arguments can easily be produced covering most of the parallel instances of the usage of wheat grains, barley grains, rattis etc in metrology.

.....How did wikipedia fall into this error?,  I believe this is a lot to do with accepting the word of incorrect authorities, in this case New Scientist, Miles and Grierson.  It would be better, in my opinion, to rely upon the alternative work of Skinner, and Morton, which I hope is usefully summarised and extended in my recent book “Early World Coins and Early Weight Standards”
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Robert Tye

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Wiki quote {from <http://tinyurl.com/yksxpzf>]:

The word [carat] came to English from French, derived from the Greek kerátion
"fruit of the carob" .... In past centuries, different countries each had their
own carat unit, all roughly equivalent to the mass of a carob seed. These units
were often used for weighing gold.

Carob seeds were used as weights on precision scales because of their reputation
for having a uniform weight. ... However, a 2006 study[3] found carob seeds to
have as much variation in their weights as do other seeds (23% vs. 25%), though
it seems that it is easier than with other seeds to recognize particularly large
or small specimens and remove them.[4] Thus, the carob seed was used as a weight
not because it was naturally more uniform in weight, but because it could be
more easily standardized.

[3] Turnbull, Lindsay, et al. "Seed size variability: from carob to carats"
<http://dx.doi.org/10.1098/rsbl.2006.0476>

[4] "Did carob seeds allow shady diamond deals?", New Scientist, page 20, 6 May 2006.
<http://www.newscientist.com/channel/life/mg19025505.100-did-carob-seeds-allow-shady-diamond-deals.htm>l