Recently, ungauged, natural-cleft `Multicolor' slates from India, South Africa, Brazil
and China have been gaining popularity as a low-cost, beautiful and practical alternative
in residential and commercial design. The rustic quality of these slates fits well with
current design trendsearthy colors, varied patterning, interesting cleft textures and a
handmade appearance provide these stones with distinctive charm in natural, comfortable
Until the last few years, most the technology of ceramic and stone industry has been
dedicated to producing increasingly thinner, shinier, more uniform tiles with less color
variation. The last few years has seen a reversal of this trend as the growing popularity
of handmade ceramic tile, honed and textured limestone, and `tumbled' marble demonstrate.
The latest crop of `stone-look' monocottura ceramic tile even have color variation, an
interesting sign of the times. Multicolor slates, which are quite rustic, and are
generally sold ungauged, are a logical extension of this trend.
The use of slate is hardly newdomestic and imported slate has always been an important
part of tile and stone masonryhowever the use of ungauged slate is now becoming a common
part of the `tile' side of stone industry, not just the `masonry' side. This has created a
fair amount of anxiety and confusion in both camps as thin, ungauged slate tiles require a
hybrid of masonry and tile methods and techniques for appropriate specification,
installation and maintenance.
I've been selling Multicolor slates, as well as other ungauged cleft stones such as
sandstone, limestone and quartzite in the Northeast US for the last few years, and I find
that there is a real need for more specific product information. While the scope of this
article is general, I'll provide some background and practical tips regarding the use of
Why are these products usually sold ungauged and loose in crates? The answer is a
matter of cost, available and flexibility. These slates are generally hand-split and
hand-cut right at the quarries, which are frequently in remote areas of developing
countries, a decidedly `low-tech' production method.
Many overseas suppliers do have the ability to gauge and box slate prior to shipment,
however, the cost of this service is generally adds 50% to 100% to the Ex-Factory price of
the stone, due to a relative lack of gauging machinery and additional handling and waste.
The results can be a mixed bag as wellalthough gauged slate does have one fairly flat
side, facilitating trowel installation, there is still a certain amount of thickness
variation, and there is a trade-off in flexibility.
Multicolor slates can be pretty wildboth in color and texture. Ungauged materials are
`cleft-both-sides,' giving you two options as a face. If you don't like one side, you can
simply flip it over and use the other side! As an alternative to supplying gauged slate,
some importers provide services here in the US to hand-select ungauged slate for
thickness, which speeds the job for the installer while still providing a choice of faces.
As obvious as this may seem, I've had a number of `panic' phone calls from installers
or distributors: "The customer says this stone has too much gold!" (Or too little
gold. Or whatever.) My first suggestion, of course, is to flip the objectionable pieces
over. The response is usually a short silence, followed by "Really?," and I
never hear about the "problem" again.
In any event, with proper planning and a little effort, ungauged material can be
efficiently used for almost any application, so when there is a choice, many feel that the
cost savings and added flexibility of ungauged slate is worth the additional effort.
Because they are a handmade natural stone product, I feel it is best to view the inherent
variation and imperfection of ungauged multicolor slates as the primary reason for their
natural charm and beauty.
Technically, slate is a metamorphic stone, composed of the sediment of decomposed stone
and organic matter, hardened by heat and pressure to the point where it does not crumble
(like shale), and yet can still be cleft (unlike schist or gneiss). Since the geological
process varies greatly, slates range quite a bit in hardness and porosity, the primary
factors that influence how a stone performs as a building material. To confuse matters
further, other cleft or split materials such as sandstone, quartzite and even limestone
are often assumed to be `slate.'
Be aware that the trade term `slate,' like the trade term `marble' is a generalization.
It is important judge the performance characteristics of different slates
individuallydon't assume that all `slates' are alike.
Most slate will perform acceptably in most common applications with proper installation
and maintenance, however take the time to consider the requirements of the job site and
learn a bit about the suitability of the slate you have selected. Keep in mind that slates
abrade a certain amount under foot-traffic and can degrade over time when exposed to the
elements, especially freezing conditions. Use the requirements of the installation to
dictate appropriate thickness, installation and maintenance specifications for a
particular type of slate. The use of certain softer slates as thin tiles for demanding
applications such as high-traffic paving or exteriors may be difficult to justify in terms
of the amount of repair and maintenance required over the expected life of the surface.
Installation Method Selection
There are many good reasons to recommend medium-bed or wet-bed installation methods for
any stone tile, but with ungauged slate, these method are the requirement. Where space and
budget considerations allow, the `wet-bed' or plastic mud bed method is preferred,
although installation using medium bed methods and a 3/4" trowel are quite
sufficient. While thin-set methods and materials are not recommended, a 1/2" trowel
can sufficient with material that has been gauged or sorted for thickness.
Wet bed methods provide the most flexibility in compensating for thickness variation,
providing an sound, even installation almost automatically, while trowel installations for
ungauged slate will generally require a certain amount of sorting and back-buttering
during installation, which can be quite time consuming. As such, the cost difference
between the two methods may be slight.
Medium bed methods require specifically formulated setting materialsordinary `thin-set'
can sag when used with a larger trowel. Most setting materials manufacturers produce a
medium bed mortar that is suitable for use with larger diameter trowels. Consult them for
Always grout slate tiles, especially on floors, to provide required support and to
avoid lippage where tiles meet. A 1/4 inch grout joint is a practical minimum and 3/8 inch
is standard, providing enough room to
compensate for variation in the size of the tiles. Where a relatively tight grout joint is
called for, a `running bond' or `brick' pattern can be used to help keep grout lines
straight, especially with slates that vary in square. Natural gray grout in almost always
preferred for slates.