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Many people think that numismatics is only a hobby, without any practical benefit. What difference can outdated, corroded, ancient coins make today? However, there is a question of critical importance in Jewish law dependent on the weight of the Islamic silver dirham in the 13 century. In Jewish law there are many ritual measurements, including the finger-breadth, hand-breadth, cubit and mile, which are measurements of length. There are also volume measurements, like the egg, reviit, kav and saah. Maimonides states how many dirham of water are in a reviit. The Talmud in turn states how many cubic finger-breadths are in a reviit.
The most authoritative code of Jewish law, Shulchan Aruch, in several places apparently follows Maimonides measurements.
Now, if we can figure out the weight of a 13th century silver dirham, we will be able to determine most capacity and length measurements.
Rabbi Abraham C. Naeh, in the early 20th century in Palestine, wrote that in the Turkish Empire a dirham weighs currently 3.2 grams. He further stated, that no one had ever commented that the dirham has changed and that the measurements should be adjusted accordingly. Therefore, Rabbi Naeh concluded, the 13th century dirham also must have weighed 3.2 grams.
Rabbi Abraham Karelitz disagreed. Based on certain 18th century responsa, who measured actual finger-breadths, he determined that volume measurements are bigger then previously thought.
He hypothesized that we can reconcile the 18th century opinions with Maimonides measurements if the Egyptian dirham in the 13th century would have weighed approximately 5.53 grams.
However, numismatists have determined that the Islamic silver dirham changed rapidly and ranged from 2.8 - 3.1 grams in the Islamic and Medieval times. So how much did the dirham actually weigh, then?
Islamic, Seljuks of Rum, Mes 'ud II (Ghiyath al-Din), 1281 - 1287/8 and 1302 - c. 1308.
Silver dirhem, weight 2.902 g,
Maimonides ruled that a reviit of water or wine weighs 17 1/2 dinars. Since the size of the gold dinar is more widely known, we can approximate the weight of the dirham, if we can determine the relationship between dinars and dirhams. Apparently, here Maimonides rounded off the numbers, since he considers water and wine to have the same density, whereas in another place he distinguishes between their relative weights.The gold dinar was also a general weight measurement. As a weight, it was known as a Mitqual (obviously the general purpose weights were not made of gold). One Mitqual weighed 4.25 grams, more or less throughout Muslim rule.
W. Hinz, determined more precisely, he claimed, that glass dinars, or mitqual, weighed 4.231 g.
Therefore, Rabbi Y. G. Weiss reached the conclusion that the dirham weighed 2.83 g. He proves this from Maimonides himself who stated that 520 contemporary Egyptian zuz (dirham) equal 346 2/3 dinar (86 2/3 selah, which are 4 Judean dinar each).
Maimonides assumes that the Judean and Islamic dinars were of the same size. Both dinars apparently were based on the Roman denarius, which in turn weighed from 3 to 4.5 grams, depending on the political situation at the time. If 346 2/3 dinars equal 520 dirham, then a dirham would weigh 2/3 dinar. So if a dinar weighed 4.23-4.25 grams then a dirham would weigh 2.82-2.83 grams, which is 2/3 the weight of a dinar. These are the official weights, in actuality rulers often coined money smaller, in order to make the production more profitable for the issuer of the coins.
It is interesting to note that in recent times several Islamic mints have minted a new silver dirham and gold dinar, in an effort to replace the U.S. dollar, though they do not necessarily weigh the same as the original coins. The modern silver dirham weighs 2.975 (or 3) grams and the gold dinar 4.25 grams. The modern coins follow a ratio of 10 dirham to 7 dinar, which apparently was followed in some regions. On the other hand, in 13th century Egypt, where Maimonides resided, it seems that the ratio was 10 dirham to 6.67 dinar, as mentioned above.
 Mishnaot Commentary, Ediot 1,2
 Talmud Bavli – Tractate Pesachim, folio 109
 Shiurai Torah, Pinhas Eben Press, Jerusalem, 1947
 Kuntres Hashiurim, in Chazon Ish, Bnei Brak, 2003, pp. 114-120
 Mishne Torah, Eruvin 1,12
 Brought by Midoth Umishkaloth Shel Torah, Y. G. Weiss, Moznaim, Jerusalem, 1984, p. 79
 ibid. p.89
 Mishnaot Commentary, Bikurim 6,15