HIST363: Global Perspectives on Industrialization

Figure 11: Porous tuff not suitable for construction

Before ‍‍the invention of standard size brick masonry, a majority of stones were custom cut by skilled artisans called stone masons. The material strength of stone was split into 6 quantitative categories: ‍‍very soft, soft, semi-firm, firm, hard and cold‍‍. Stones in the soft range often included less compact tufa which were widely used in construction for their ease of cutting while marble and granite were considered cold and used by the elite class and to decorate the facades of buildings. Stone masons did not categorize stones by material make-up, but by the purpose they would serve. This system worked best because the hardness of stones is not consistent ‍‍across the board‍‍, there are "plenty of limestones that are harder than certain marbles, and some marbles have to be worked a little like granites".

For this reason, color was not used often to identify stones because it did not help masons identify its purpose. Categorization by purpose rather than composition is evident even today in the case of Purbeck Marble in Britain, where it is actually is a limestone. The modern name comes from long ago when Romans invaded Britain for its mineral wealth and classified it according to its purpose.

Ability to take a finish was an important consideration for stone masons. Softer stones could not hold as much detail because their surfaces wore away easily while colder and harder stones could hold a high amount of detail. The Latin word 'marmor' refers to a stone's property of being able to take a polished finish, stone masons often used this word when classifying stones.

If the local supply of stones was diverse, masons could designate certain types of stone for specific applications. For example, it was important to not use porous tufa as a structural element if it would be exposed to weathering. Vitruvius notes the pros and cons of soft tufa: "the stones which are not hard have the advantage that they can be easily cut and are good when used in covered places, but placed out of doors, the frost and rain turn them to dust…".

In other words, the water that infiltrates the pores causes freeze-thaw cycles which rapidly wear the stone. Vitruvius recommends quarry sapping as a factor of safety before using tufa. This method involves stocking quarried stone in an outside and covered location for a couple years. The quarrying had to be done in the summer so that the stones had minimal moisture content. Stones that wore too much did not make the cut and were crushed and used as aggregate.