An Introduction to
Ungauged Natural
Cleft Slates

Peter F. Galgano

Originally Printed in Stone World Magazine, June, 1994


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 spaces.

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 these stones.


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 Speaking...

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.

Material Selection

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 specific recommendations.

Grout Selection

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.

Installation tips

bulletUngauged slate usually requires additional preparation to insure proper bonding and a smooth, trouble-free installation. Plan to inspect, prepare and sort ungauged slate as necessary before setting tiles. Brush or rinse dust and dirt from the surface to be installed, remove any loose scale or clefting with a stiff nylon brush or a putty knife, and remove cracked or unsound pieces, reserving them for cutting.
bulletTo insure maximum support of cleft slate, particularly in trowel installations, it is important to fill clefts, and voids by back-buttering individual tiles, and try to achieve 100% mortar coverage.
bulletUse care to avoid flash-curing of setting materials and grout, using craft paper and/or misting the surface as required. Slate tiles are porous and can draw the moisture out of a setting system, particularly in warm, dry environments and where sunlight or radiant heating raises the temperature of the stone before the system has fully cured.
bulletThe sealer that will be used for maintenance can be applied as a grout-release when necessary.


Proper maintenance of slate is similar to that of any stone surface. There are two basic methods of maintaining slate surfaces, surface coatings or penetrating sealers. While you can switch between the methods, this can be tricky and expensive, so it is better to consider the options and specify the maintenance from the beginning.

I am a proponent of subsurface sealers for most stone surfaces. My testing, while not very scientific, reveals that any good quality penetrating sealer designed for stone is big help in ongoing maintenance of slate. I especially like the latest generation of `silane' sealersthey genuinely repel most water and oil-based stains on slate and grout.

Topical sealers have advantages and drawbacksthey can be difficult to apply, and require periodic repair, stripping and reapplication using strong chemicals, but coatings are the only means available to protect the slate from traffic, add shine and enhance the color.

I recommend the use of surface coatings to maintain slate in just three cases:

  1. As a traffic barrier used to protect softer slates in commercial settings;
  2. As traffic barrier to protect slates where the color is only a thin layer on the surface, such as African Multicolor Slate;
  3. When shine or color enhancement is required.

There a quite a few systems of surface coatings, however I would generally stick to acrylics, which are durable, repairable and stable. Urethanes, varnishes and other processes are usually poor choices for slate.

Maintenance tips

bulletAlways test maintenance products prior to use. Avoid using strong cleaners or chemicals on slate, including acids, alkalis and bleaches, unless you have tested them on your slate, and you are confident in what you are doing. Use products formulated for use on stone. Consult your stone maintenance products supplier for specific recommendations.
bulletApply sealers only once the setting system has had a chance to fully curea couple of weeks or more is ideal. Attempting to seal a damp surface can be rather unproductive and messy as the moisture prevents the sealer from penetrating into the surface.
bulletSome slates will dust for a while after installation on floors as loose scale is removed by foot-traffic. This will stop as the surface `settles-in,' usually in a few weeks or so. Keep the floor well swept to avoid unnecessary abrasion, and reapply sealers a month or two after installation to insure thorough coverage. Reapply sealers every year or so insure maximum protection.
bulletOngoing maintenance is quite simple. Keep the floor clean and well swept. Use dilute, pH neutral cleaner or stone soap, rinse with clear water, and buff with a soft cloth to remove cleaner residue. From time to time, scrub grout and clefts with a stiff nylon brush and rinse to remove built up dirt.