Weathering of slate

A good slate can be considered as a very durable stone and resistant to weathering. There are no comprehensive investigations known to the author which allow reliable statements of the weathering behaviour of slate.
The influence of the single weathering processes to the stone are often hardly to quantify so that a final reliable classification is a kind of a qualified assumption. Roofer and customer want to know the life span of the roofing slate but it is very unlikely or impossible to make statements whether the functionality of the roofing slate is given for a certain time.

The regional climate and more the environmental conditions gives information of the general weathering processes but the microclimatic conditions in and at a building can be very complex. Roofs, for example, are clearly exposed and can dry up after a rainfall. In niches moisture can remain and together with air pollutants induces the accumulation of salt and finally damages by salt action. In more or less humid places and below trees an intense biocolonisation can occur.

Besides for roofing, slate has been also used as dimension stone. Problems with deterioriation and the question of possible conservation measures of valuable objects could arise but slate has no real importance in conservation science compared to other dimension stones.

It is common practice in conservation science to firstly map the deterioration and damage pattern to get an overview of kind and intensity of the different deterioriation pattern. An applicable guideline for mapping and classification is the illustrated glossary on stone deterioration as an international established standard published by ICOMOS-ISCS (International Scientific Committee for Stone). Using this glossary guarantees a common language among professionals. Thus, if talking about weathering phenomena and damage phenomena one should use appropriate terms.
This glossary is freely available as PDF on the website of the ICOMOS in different languages.

The glossary comprises six families and each of it comprises 2 to 11 terms. The first family comprises the general terms in respect to weathering and stone. The other five families are crack and deformation, detachment, features induced by material loss, discolouration and deposit, as well as biological colonisation.

General terms after ICOMOS

• Weathering: Any chemcial or mechanical processes in stones which are exposed to the weather and experience changes in character and deteriorate.

• Alteration: Modification of the material that does not necessarily imply a worsening of it characteristics (example: reversible coating). A damage is a human perception of the loss of value due to decay.

• Decay: A chemical or physical modification of the main important properties of the stone, leading to a loss of value or to the impairment of use.

• Degradation: Decline in condition, quality or functional capacity.

• Deterioration: Process which lead to a worsening or lowering of the quality, value or character of the stone (depreciation).

These terms are objective: Biocolonisation by algae, for example, is an alteration irrespective of the customer which could consider it as degradation.

Pictures

Exemplary images of alterations and damages of slate after ICOMOS. Move mouse over thumb to enlarge.

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  Fig. 1 Flaking

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  Fig. 2 Exfoliation

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  Fig. 3 Flaking and Exfoliation

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  Fig. 4 Discolouration

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  Fig. 5 Discolouration caused by fixing hooks

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  Fig. 6 Spalling

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  Fig. 7 Efflorescence caused by carbonate in the slate

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  Fig. 8 Efflorescence caused by the mortar

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  Fig. 9 Patina

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  Fig. 10 Biological colonisation