Get more from your slate investment.

Get more from your slate investment.

Aging processes

"In time, and with water, all things change." -Leonardo da Vinci

Water's central role

Water itself is not typically injurious to building stone; the processes that it enables are. The most effective way to prolong the life of any building stone, including slate, is to limit exposure to water.

Petra Petra

Without water to activate aging processes, Petra's soft sandstone reliefs remain sharp after 2000 years in the Jordanian desert.

Four principal processes age building stone in general, and slate in particular:

  • mineral crystalization
  • oxidation
  • freeze-thaw cycling
  • acid attack

Each of these processes is described briefly below. Without water (and, in most cases, free oxygen), these processes cannot occur.

Mineral crystalization

Slates can contain calcite and other problematic minerals. Water permits a chemical reaction that converts calcite into gypsum. Gypsum grains are larger than calcite grains. Newly formed gypsum grains press against the slate's substrate, stress it and weaken it, leading to material fatigue and breakdown.

Mineral crystalization


Water does not act alone to age slate. It must be accompanied, for example, by freezing temperatures, or acids, or free (chemically unbound) oxygen. Slate in the ground is generally wet, but without these other agents, it remains stable. Upon quarrying, the various aging processes begin. With exposure to air, some slates are particularly suseptible to oxidation.

Slates often contain minerals commonly (and sometimes incorrectly) referred to as pyrite. Water and free oxygen cause some of these "pyrites" (and other minerals) to oxidize. In the early stages of oxidizing, pyrites swell and stress the slate's substrate, leading to material fatigue and breakdown. In time, pyrites can oxidize away and leave voids. Voids invite deep water penetration, hastening other aging processes. Some voids become holes.

Oxidation Oxidation

Freeze-thaw cycling

Water permits freeze-thaw cycling. Expansion and contraction in small fissures and pores can cause material fatigue and breakdown.

Freeze-thaw cycle

Acid attack

Acid attacks when it is in solution, that is, when water is present. When stone dries, acid attack stops.

Acid attack

By allowing slates to dry quickly, SlateSpacer™ shuts down aging processes.