Slate quality question...

Tomster

Registered
Hi Guys,

I'm wondering if the whites lines running through this (Chinese origin) slate are a bad thing - do they make the slate more likely to crack or chip?

Be good to know what they lines actually are, I've seen them before but in a more random pattern, not straight across from one side to the other like in the photo.

S__32129092.jpg

Thanks in advance.
 

trentfromtoledo

8onthebreaktoledo
Silver Member
Hi Guys,

I'm wondering if the whites lines running through this (Chinese origin) slate are a bad thing - do they make the slate more likely to crack or chip?

Be good to know what they lines actually are, I've seen them before but in a more random pattern, not straight across from one side to the other like in the photo.

View attachment 518223

Thanks in advance.

I did some quick research and came up with lines of Quartz. Just what I found. Anyone else have input to confirm or add?

Trent from Toledo
 

tjohnson

AzB Silver Member
Silver Member
Hi Guys,

I'm wondering if the whites lines running through this (Chinese origin) slate are a bad thing - do they make the slate more likely to crack or chip?

Be good to know what they lines actually are, I've seen them before but in a more random pattern, not straight across from one side to the other like in the photo.

View attachment 518223

Thanks in advance.

Those slabs are metamorphic rock, probably something close to slate. The lighter bands are layers that are slightly richer in feldspar and quartz (good job Trent!). The darker layers also have quartz and feldspar, just not as much.

The problem I see is how the slabs were cut. Typical slate is cut so that the slabs are parallel to the natural layering in the rock (known as "foliation"). This makes sense because the foliation planes are natural planes of weakness. That's why slate naturally splits along these planes. On these slabs they cut the rock perpendicular to the foliation, so the planes of weakness are running vertically through the slab which is not good, in my opinion. I think these slabs are going to be much more prone to breaking than slate that is cut parallel to foliation.
 

Tomster

Registered
So we managed to break one, but not in the direction of the "foliation", if I have the term right...

:banghead:

5673.jpg
 

tjohnson

AzB Silver Member
Silver Member
So we managed to break one, but not in the direction of the "foliation", if I have the term right...

:banghead:

View attachment 518345

Yep, they'll break that way, too. depends on which way you bend it.

And now that I can actually see the rock up close, it's not slate. Looks like a coarser-grained type of metamorphic rock called gneiss. The foliation surfaces in a gneiss are stronger than in a slate so these slabs might not break as readily along the foliation planes.

Slabs made of this kind of rock lack the flexibilty of slate slabs. Slate can be shimmed without breaking because it's a little bit flexible. The flexibility is also enhanced by the foliation being parallel to the slab. Sandstone slabs ("brunstone") and slabs like the gneiss that you have are much more brittle, especially if the foliation is perpendicular to the slab.
 
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