On page 70 of the aforementioned they discuss a series of tests conducted on a section of Warrior's hull removed when she was made into a jetty, but stored aboard. On testing something unexpected was observed, the armours resistance to impact is highly dependent on the temperature:
Temp (degrees C): Absorbed Energy (j)
0: 11
15: 17
40: 24
100: 26
The observation that iron is less ductile and more brittle at lower temperatures is not a new one, but the degree to which protection changes is astounding. The armour offers twice the protection in the tropics than in the arctic. Thus Lambert concludes that the temperature, weather &c. all play a role, and this can explain the erratic results of armour tests.
My own observation is that it implies armour is probably more effective in a steaming warship (which is kicking out a lot of heat) than on the firing range.
In the ACW it implies that the ambient temperature of the armour is a major factor. The operating temperature in a Monitor turret was pretty high, so I guess the armour was near full strength. This bears further investigation.
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During the ACW, metallurgy was not highly advanced. They couldn't figure out, for example, why some cannon tubes ruptured and others did not. Trial and error led to breakthroughs in seasoning and the like, but without scientific instruments to evaluate grain structure they were pretty much guessing.
It would be interesting to see a spectrographic test of the monitor (and other ACW) iron to see what the chemistry is. My hunch is that iron recipes will react differently to temps, and that the iron going into monitors and ironclads (being sheet) will not be exactly homogeneous anyway.
Modern irons and steels have many, many grades to satisfy the customer requirements for ductility, machinability, wear resistance and the like. Metallurgy to me is still somewhat of a black art, as much learned on the floor as out of books. But, I'm a complete layman just looking over their shoulders.
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