How did they do it?
THere are certainly a variety of methods for subtractive work with stone. The majority of the situations involving working marble are pretty straightforward.
There is a pretty basic set of tools involved. Sets of hammers and chisels for rough, medium and fine work. Usually the hammers are very heavy, of either metal or wood (mallets). The chisels are of varying lengths and thicknesses, and usually have a different blade depending on the type of cut you want (i.e. pointing, lining, blocking, etc.)
The difficulty in working with marble is not so much in the possibility of cracking or splintering, as marble (particularly Carrera marble) is actually quite “soft;” That is to say, it is workable as opposed to granite or stone with higher metallic/oreic concetrations. Marble has a high concentration of very fine silicates combined with a very tight molecular “lattice” which makes it possible to remove very small amounts of material along almost any axis without fracturing it along a plane in any given direction.
The real difficulty and danger in working marble (or any stone) is the repetetive motion with the use of the tools. It is VERY VERY tiring. Swing a 5lb. hammer against a 5lb., 2 foot long steel chisel in a 12" arc about 500 times, and you will see my point. The material is actually pretty easy to remove. The danger in ruining a work is that your arms and hands get very tired, and it is quite easy to miss a swing. Compound this with the fact that progress is extremely slow, and you have yourself a perfect opportunity to screw something up with a careless swing. The individual blows are not actually very hard. Most would assume that you have to really smash the h*ll out of it to remove some material, but thats not the case at all. It’s more like thousands of tiny swings like a jackhammer that removes a tiny bit at a time. DUring the process it is necessary to switch to various degrees of chisels:Blunt pointers and short blades to go from blocking or roughing out an area, liner chisels (with teeth) to shape and increase detail, and then very fine smaller liners and rasps/files with and without teeth to polish the details prior to actual finish polishing.
To rough out or block out the designs, usually you start with a square block, and draw the image on each face of the block as it will appear when looking directly at that face. Then, while removing larger chunks with a heavy pointer or bladed chisel, occasionally going back in with charcoal and redefining the drawing.
Hope this helped.