Weights and Measures

One of the problems with attempting to produce Calotypes from the original texts in particular the English ones is the unfamiliar system of weights and measures.
During the Calotype era of the 1840s and 1850s  when the process was at its most popular , there were three scales of measurement !
These were
Troy weights , commonly used for measuring precious metals and gem stones.
Avoirdupois , the system used for common merchandise
Imperial measures , for fluids.
Troy Weights
Pound    ounces   Drachms   Scruples   Grains   Grams
1              12            96             288           5760      372.96
                   1              8                24            480        31.08
                                     1                 3               60          3.885
                                                         1               20          1.295
                                                                              1         0.06475
Avoirdupois Weights
Pound    ounces     Drachms   Grains                       Grams
1              16              256            7000                        453.25
                   1                16               437.5                     28.328
                                        1                  27.34375             1.7705
1 0.06475
Imperial Measures
Gallon      Pints        Fluid ounces    Fluid Drachms    Minims    millilitres
1                8               160                   1280                     76800      4546.09
                   1                20                      160                       9600        568.261
                                         1                          8                         480         28.413
                                                                      1                           60            3.55
Presumably by the time they left school the Calotypist had had all of this drummed into them . In none of the manuals (with the exception of one )I have read does it specify which system of measurement to use . Being a child of the 60s I had just about grasped the imperial monetary system when decimalisation came into place and the difference in weights and measures happily sailed over my head until I started making Calotypes . I had never heard of the grain and will confess to being somewhat bemused at the thought of counting grains of chemicals ! Luckily this is the 21st century and we have the Internet.

The grain was the least ambiguous of the English Weight Units because it was the common denominator of both the troy and avoirdupois systems. And studying the various manuals of the time the chemical denominations are usually in grains , sometimes drachms and very rarely scruples . Fluids are usually ounces or drachms , some times pints or quarts (2 pints) and very occasionally Minims. 

So it would appear by the inclusion of scruples that dry chemicals are measured using the Troy scale . This is confirmed by Robert Hunt in the appendix of his 1853 Manual of Photography. Where he both confuses and clarifies the situation by listing the Drachm under the Apothecaries scale 60 grains to the drachm and helpfully 3.8864 grams . Of course the Apothecary - a person who sold medicines and drugs was the predecessor to the modern chemist . And there were several chemists who contributed to early photographic improvements.  Robert Hunt , Chas Long and C J Jordan being among them . Also there was Alexander Govan of Smith and Govan the chemists 3 doors down from Dr John Adamson’s house and who’s priceless album of early prints still exists in the Special Collections at St Andrews.

Of course in the mid nineteenth century scales were brass with small brass weights , where as today accurate  digital jewellers scales are available very cheaply. The measurement of chemicals in the 1840s and 1850s presumably  had the potential of being as unpredictable as the purity of the chemicals.

Here in the 21st century it is often useful is often useful to express chemical measures in percentages .
To calculate a percentage from one of the old manuals. First convert to metric .
The Chemical Quantity x 100 / the quantity of water.
Or to calculate the percentage of something ie 10% of 1 fluid ounce in mill.
28.41 x 10 / 100 = 2.84 mill.
This is very useful when comparing processes.


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