roofing slate dictionary - The hardness of a slate

The hardness of slate

The hardness of a rock mostly means the hardness in respect to the wear (abrasion), the scratch and corrosion. Especially the first one is important for rocks which are used for flooring.

There are several methods for the determination of the hardness of a mineral. Among all the Mohs scale is the most common method but should be considered as a field method. The scale includes several minerals with a different hardness and the scale ranges from 1 for talc to 10 for diamond.

More exactly and described in form of numbers is the determination of the crushing hardness also known as Vickers- , Rockwell or Brinell hardness. The hardness of a slate depends on the hardness, splitability and the amount of the different slate building minerals as well as from the fabric that means their spatial orientation to each other. Compared to other rocks like granite slate is suppose to be a relatively soft rock.

For roofing slates the hardness is not so important. Yet, it is possible to distinguish the production hardness in respect to the mining, producing and finishing on the roof. The production in the quarries is very mechanical and thus little differences of the hardness are hardly discernible. In contrast, the roofer while working with his hammer, is able to notice also little differences of the hardness. Because of this, for the estimation of the 'finishing hardness' also roofers were consulted.

Vulpius (1993) classified the slates in several groups according to their quartz-mica relation. This groups were then modified in more groups by Baum in 1994 (see table). The basis for this useful division is the analysis of the mineralogy of more and different international slates, although Thuringian slates still dominated.

Following analysis mainly of Spanish slates (Wichert, 1997) could slightly balance this distortion and give of deeper insight in the mineralogical composition of roofing slates (see diagramm).

Group	Relation Quartz/Phyllosilicates
	Vulpius	Baum
very quartz-rich slates	-	> 1,25
quartz-rich slates	1,25 bis 0,77	1,25 bis 0,77
quartz-emphasized slates	0,77 bis 0,5	0,77 bis 0,5
phyllosilicate rich slates	< 0,5	< 0,5

Gruppe	Relation Quartz/Phyllosilicates	Properties
very hard slates	> 1,3	high effort during punching and cutting of the slates by means of a hammer it is partly impossible to process the slate with a hammer due to the extrem brittle behaviour of the slate, leading to breakage or splitting
hard slates	0,9 bis 1,3	higher effort during punching and cutting of the slates by means of a hammer possibility of easy splitting
medium hard slates	0,5 bis 0,9	normal effort during punching and cutting of the slates by means of a hammer
soft slates	<0,5	easy punching and cutting splitting surface feels slightly soapy

Division of slates according to their quartz-phyllosilicate relation (upper table according to Vulpius and Baum) or modified according to their hardness, respectively (lower table).

This fact and the knowledge about the 'finishing hardness' especially of Spanish slates modified again the classification of Baum and is described in the table. In this modification the amount of feldspar is included because its hardness is more or less equal to quartz.
In addition, the very fine grained texture of slates reduces the influence of the higher splitability of feldspar compared to quartz should not have a reconstructable influence on the mechanical behaviour.

The aim of this hardness classification is to enable a person to make statements about the 'finishing hardness' and to the general 'finishing behaviour' of a slate. Further mineralogical investigation, for example, of slates in the US and China could certainly complete, enhance or change this classification.


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