I am happy to share my knowledge and experience of French slate and marble clocks with those who are interested. My knowledge has been gleaned over the past 4 years or so and have text books on the subject.
I am a collector of French slate perpetual calendar clocks at the moment and even just calendar clocks. I find the movements fascinating. A good book to read, though all in French, is 'Les Brocot - Une Dynastie D'Horlogers' by Chavigny. A non-technically minded friend has had a good stab at translating the chapter on 'Calendriers' (Calendars) and I've had a go myself. However, I am still looking for someone who is a horologist as well as understanding the French language to better translate it. One of the main reasons of course that I find these clocks fascinating is, as long as the clock itself is kept wound, the perpetual calendar will show the correct date, day and month year after year as it can adjust automatically for the short months as well as Leap Years, except those not divisible by 4 of course. 1900 was the last one not recognised, but 2000 would have been handled by these calendar movements.
It is, in one of the better quality calendar movements by Brocot, a wheel divided into 4 sections which is responsible for the accurate month and date indication and acts like an analogue programme (that you would find in a washing machine for example) that 'tells' the date hand to 'knock-on' on short months and also of course from 28/29 February all the way over to 1 March. It is the series of 22 notches, as well as the smooth parts, in this wheel in the calendar movement that dictates what date the hand points to and, consequently, the month. There are other styles of perpetual calendar movements but it is this 'analogue programme wheel' that determines whether a calendar movement is perpetual or not. On most movements I have seen this wheel is obvious if one looks at the calendar movement, although in certain clocks the calendar mechanism is hidden and only by manual correction of the calendar would one know if it was perpetual or not. For me, this would be crucial to determine before purchase.
I would love to own a gilt brass, 4-glass example with fly-back date hand and there are even French table regulators which have a full perpetual calendar and also with times of Sunrise and Sunset. Can you imagine what the gearing must be like to handle this sort of read-out?!
On a more general note re French slate 19th century slate cased clocks, it is often a sad fact that the edges of the cases have been chipped - some small ones, but also some more serious ones. The minor ones should really be left but the larger ones, in my experience, can be restored by a competent stonemason. The difficulty with smoothing over the smaller chips is that the substance used does not adhere well to the existing 'good' slate and, when it gets buffed up and smoothed, often just comes off again.
Please feel free to ask questions and I will do my best to help you. It could be you could help me if there are any more knowledgeable 'clockies' out there!
Thanks for reading this far.