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Maintaining Slate Floor Coverings

May 19, 2004
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Slate is a metamorphic rock. It begins as fine-grained clay mud. The process of extreme pressure and heating causes the mud, composed of sediments of decomposed stone and organic matter, to first to turn to sedimentary shale and then to slate.

Slate cleaves along flat planes and is split into parallel sheets. They are easily identifiable by their "cleft" irregular surface; however, they can be honed to a smooth surface resembling a chalkboard.

Slate is primarily composed of clay-like materials, which usually make it very soft and susceptible to scratching. Slates generally have a very dull look unless they have been sealed or have a chemical coating on them, and they vary widely in hardness and porosity. Because the geologic processes of metamorphism may vary a great deal, the hardness of slate will vacillate somewhere between 2.5 and 5.5 on the Mohs Scale; this gives them a wide range of hardness. Similarly, the porosity of the stone will vary as a result of these same geologic idiosyncrasies.

Slate is generally uniform in color, and is frequently found in dark colors of gray or blue-gray and black, but may also be green, red, brown or yellow. Slate also has many types of multi-colored stones from various parts of the world.

The properties of slate allow it to be virtually invulnerable to attack by most common chemicals; therefore, maintenance of the product is relatively easy. Most cleaning chemical solutions can be used safely on slate and will have very little impact on the slate itself. Strong alkaline or acid solutions may not affect the slate, but may have some overall impact on the grouting.

Initial Maintenance
Initial maintenance for slate floors begins with installation. As the material is installed, it is important to wipe off excess mortar with clean water. This prevents the accumulation of mortar and mortar stains from forming on the surface of the slate. The water used to wipe down the surface should be changed frequently to ensure excess material is not left on the surface. If the water is dirty, residue hazing may occur, which can be extremely difficult to remove after it has dried. This is especially true when using latex or epoxy grouting material.

Initial Cleaning to Remove Contractor Soils
After a period of time (at least two weeks), the slate should be set. At this time the floor should be scrubbed using an approved cleaning chemical and appropriate pad (blue/green). Generally, neutral cleaner is acceptable, but on some occasions an all-purpose cleaner may be applicable to remove more stubborn soils. If mortar stains are present, a mild acid wash may be in order (if this condition exists there are specialty stone companies in every city that can help you in selecting the right cleaning product for your condition).

Rinse the floor thoroughly and wipe up standing water with soft toweling or a chamois cloth to eliminate potential water stains.

Application of Penetrating Seals or Impregnators
Although slate is impervious and really needs no seal or treatment, the joints and the grout lines may. Additionally, by adding an impregnator, seal or coating, the maintenance of the floor surface will be much easier to perform in terms of less mop drag and employee fatigue. Use of chemical coatings on slate floor coverings will generally cause the stone to become darker in color and, in the case of sealers, provide a glossy appearance.

Sealers for slate can be solvent-based or water-based and there are many varieties to choose from. Sealers that are best suited for slate floor coverings possess less than 25 percent solids and have a low viscosity. When sealers (either solvent or aqueous) are used, a thin film is left on the surface, creating gloss that may range from satin to high gloss.

Impregnators generally do not leave a film on the surface and therefore possess very little gloss. They do, however, penetrate into the body of the slate and, more importantly, the grout to protect it from water infiltration, soil, and to make cleaning easier.

Impregnators are different than sealers; follow the manufacturers recommended procedures precisely. In most cases the wiping up of excess impregnator is essential; it also causes the biggest problems if it is not done. Read the instructions carefully.

Aqueous floor seals and finishes traditionally associated with resilient floor care can be used on slate floor coverings. Because of the irregularity of the cleft stones, no-buff systems are generally the best. Multiple coats of floor seal and/or finish may be required to achieve the desired gloss level. Spray-buff, high-speed and ultra-high-speed systems, for the most part, do not work with slate floor coverings because of the irregularities in the surface. Unlike sealers and impregnators that are rated to last multiple years, aqueous floor seals and finishes will not last as long and require a more aggressive floor maintenance program.

Daily/Routine Maintenance
As with all hard floor surfaces, daily/routine sweeping or dust mopping of the floor keeps dust and grit from accumulating. The frequency of the dry service procedure will fluctuate depending on the assessment criteria. Environmental and traffic conditions in the facility dictate the frequency of the service. In some situations, multiple repetitions of the dry service procedure throughout the day may be required.

Natural stone floors should be wet-mopped with neutral cleaner or approved stone maintenance cleaners on a daily/routine basis. Manufacturer dilution ratios should be used accordingly. The classification of the mopping procedure will be determined by the type and amount of soil to be removed. Of course, all liquid spills should be spot-mopped as they occur.

Remove superficial soil by damp mopping. Moderately soiled floors can be cleaned using wet mopping. When the floors become heavily soiled, the aggressive mopping service procedure will be required. Strategically scheduling the different wet-mopping service procedures has the potential of extending the periodic cleaning schedule.

Periodic Maintenance
In many situations it may be necessary to machine-scrub the floor on a periodic basis. When the mopping procedures no longer produce the results desired, machine-scrubbing may be the option, generally the light-scrubbing service procedure is used in lieu of wet mopping in large areas, using an automatic scrubbing machine. Medium scrubbing will remove most soils; however, the heavy-scrubbing service procedure may be required for more stubborn soils. Synthetic pads or brushes are usually safe to use on slate floors, with the exception high productivity pads and stripping pads; choose the softest pad that will perform the service you desire.

There will be times when a slate floor may have been sealed and/or finished. If the stone floor is using an aqueous chemical-coating system, the machine-scrubbing service procedures may be required periodically. The classification of the scrubbing service procedure to be scheduled will be dictated by the areas soil conditions. Always rinse the floor well after any periodic cleaning to remove any detergent or alkali residue.

Additional coats of floor finish may need to be applied after the cleaning when aqueous floor seals/finishes are used.

Salvage/Restorative Maintenance
Over time, all floor coatings will begin to show wear. A traffic pattern will appear that may appear like the slate is being worn down. When this happens, it is usually the seal, impregnator or finish wearing away exposing the slate under it. Thorough cleaning and reapplication of the product will be necessary. It is important to know what is already on the floor, because chemical coatings are not always compatible; contact the manufacturer and establish what needs to be done. If the floor product is not known, it is best to remove the old seal/impregnator completely and start over.

Again, if the stone floor has aqueous resilient floor-coating chemicals on it, it will have to be removed using the stripping-and-refinishing service procedure. The procedure is the same as removing old finish from resilient floor coverings with the exception of using a less abrasive pad. Once the old finish has been removed, multiple coats of finish may be reapplied, or the system can be switched to a penetrating sealer or impregnator.

If penetrating sealers or impregnators are used, the salvage/restorative process may only be required every two to five years, depending on the product. The frequency of the stripping-and-refinishing service procedure will depend on the chemical coating and environmental conditions the floor is subjected to. Most strip-and-refinish situations on slate floors can last one to two years, in some situations even more.

Understanding the composition of slate floors can make a huge impact on your floor maintenance program. Before taking on slate floor coverings, research the classification to identify the specific type in your facility. Then talk to the professionals at your local stone distributor about the best maintenance methods available and products that they carry. Once you have all the options, present them to the customer and suggest a course of action.

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