Rajaraja Chola (985-1014) invaded Lanka in 990 AD and conquered the northern half. Lanka regained independence from Chola occupation in 1070 under Vijaya-bahu (1055-1110).
The design and nature of the traditional Lanka type massa is the
same as the older coins issued by RajaRaja Chola and probably used
the same process of manufacture and metal composition
Obverse : Standing king with torch on left and group of spheres on right, surmounted by crescent. Reverse : Seated king on left facing right with legend beneath his raised arm. Sri RajaRaja in Devanagari script
I obtained a lot of 100 RajaRaja Chola Copper massa
Mainland type from a reputed dealer in USA. It was an unopened package
obtained from an Indian source and was probably packed in Bangalore
in December 2002. They appear to be a random sample from a larger hoard.
For each coin I measured 3 quantities using a digital caliper and scale
1) Diameter to 0.01 MM (across the width of the Obverse)
2) Thickness to 0.01 MM (middle of the coin)
3) Weight to 0.01 grams.
In the graphs above, I show the distribution of the measured parameters. The smooth curve is a generalized histogram which does not require binning the data and highlights what is significant.
The distribution of the diameter appears to be bimodal and I therefore divided the sample into coins smaller or larger than 17.5 MM. The mean and root-mean-square (rms) dispersion is given in the table for each subset of the sample. The coins with smaller a diameter are thicker than the coins with larger diameter but on average both have the same weight.
In the graphs above I show the covariance of the measured parameters. Two different symbols are used to identify coins from the two samples. The table gives the correlation coefficient in each case. The graphs on upper left and right shows respectively that larger and lighter coins are systematically thinner. The graph on lower right shows however that the diameter of the coins are not correlated with the weight. This independence is unlikely if cold planchets were struck to make these coins.
IMHO this indicates that the coins were hot-struck from a cooling drop of molten metal. The drops of metal probably had a rms scatter in weight of about 8.5%. The scatter in the force of the strike is independent of the scatter in the weight of the drops of metal. A more forceful strike would spread the molten metal more and make a larger coin. If the diameter of the resulting coin is purely a function of the force of the strike, that would then result in the observed independence of diameter with weight. The resulting thickness of the coin would be correlated with weight and anti-correlated with diameter as expected.
I should have recorded the die-axis and variety at the same time. The variety in this case is not an issue since practically all were of same type. The distribution of die-axis estimated visually to nearest 30 degrees is as follows.
The die axis shows a preference to be aligned like medals and modern coins with a smaller tendency to use the standard alignment used in older coins. The cross alignment is less frequent and probably caused purely by scatter from the strike.
I would appreciate getting comments on this explanation for the observed lack of correlation of the diameter with weight of the coin. Has anyone determined how these copper massa coins were struck in medieval India. Any reference would be much appreciated.
Osmund Bopearachchi informs me that moulds for casting planchets for massa coins have been found in Pollonaruwa. It will be interesting to do a similar study on Lankan massa coins
Alan Van Arsdale agreed they are hot struck, suggested an alternate explanation. Cast flans struck before they cooled. Taken in batches, later ones being cooler from sitting longer so smaller diameter. i.e. replace the variation in the Force of the strike with a variation in temperature of the cooling planchet.
Rudy Boekel, secretary of Sri Lanka Numismatic Society said, I think you did a fantastic job analysing all and it looks definitely hot stuck to me too.