Clean Slate (January 2008)

Along with the variety of colors and textures, slate brings with it a variety of maintenance problems that can be the bane of many new hands in the world of caring for stone. But, it doesn’t have to.
Slate is a beautiful product. It has strong rich tones, natural ridging and a real earthy look. The problem with the material is two-fold and comes down to what really is slate and what’s sold as slate. It also rests on the general janitorial perception that “all stone was created equal.”
Let’s first address what slate is, and what it’s perceived to be. The MIA Dimension Stone Design Manual (DSDM) Version VII defines slate as:

a fine-grained, metamorphic rock exhibiting “slaty” cleavage, which allows it to be split into thin sheets. It is a lowgrade metamorphic rock formed from shale, which is a thin-bedded, fine-grained, clastic sedimentary rock compacted from mud of clay-sized silicate clay mineral.

Now for the layman (myself included), I define slate as compact mud. How slate is formed (and where it is formed) plays a big part in the quality of the material.
Essentially (again refer to the MIA DSDM Chapter 10 for detail), slate is formed by these clay-like minerals settling underwater and eventually being compacted by the sheer weight of the body of water.
If you believe in the flood of Noah’s day, you can easily understand how so much slate can be found in so many different densities and textures all over the world and compacted so near to the Earth’s crust. (If you don’t prescribe to that belief, feel free to research the many possibilities available as to where it came from.)
Stone treatment, though, isn’t part of the theological argument. Your maintenance and cleaning of slate is dependent on the stone’s geographic origin and the local minerals that contributed to its makeup.
Why? Because, in this stone nerd’s experience, you can have a slate floor made up of about four general colors and only one of the four of them will be absorbent – or two of the four, or three out of four, or even all of them. This is because not all clay minerals¹ are equal, nor is the amount of compaction that it was subject to in its formation.
Think of it like making snowballs. The tighter you squeeze, the harder and more dense the snowball. If you use wet snow, it creates a denser, heavier ice ball the tighter you compact it. Your consistency all depends on the type of snow (fluffy, compact, slushy, etc.) and the amount of compaction.
Now that we’ve determined that slate can vary in make-up from one location to the other (or even within the same quarry, depending on formation), we realize that each maintenance program must take into consideration the properties of the slate at hand (or at foot, if on the floor).
Personally, I like slate in its natural state, unfettered by man’s chemicals and the abuse of maintenance cycles. But on the other hand, not everyone has my style, charm and taste. Therefore the need for sealing, coating and/or color enhancing the slate jungle has become a popular choice for many.
There also comes, with one maintenance method, a drawback against changing one’s mind down the road. Let me explain.
When I enter a dwelling with a beautiful Vermont slate (a generally dense slate) installation with its blue, green and reddish tones in place for 20 years or so, I tend to see something other than the stone itself. Nine times out of ten, it’s been coated several times with either a commercial acrylic sealer or – sigh – the worst acrylic products known to man, sold from the shelves of your local grocery store.
As much as I’d prefer to strip it and leave it bare, that won’t return the beautiful natural look of 20 years ago. Acrylics are nasty to strip and will leave white residue (especially in the clefts) that, in many cases, will never come off without using a method that will damage the finish. Our only way of maintaining this finish is to essentially strip and re-coat.
Also, on the softer slate finishes, you get situations where coatings will soak into some of the more-porous areas and never come out. In both of these situations, you also have the grout to contend with as it holds old waxes and looks unsightly.
With that in mind, try to avoid coatings on a new installation of slate unless the owner wishes to maintain the surface, with these products, on a regular basis.
When preparing to devise a natural-look maintenance program for slate, impregnating sealers and color enhancers work very well if applied properly. Remember, though, if applied as a coating, your problems will be even worse than the ones with acrylics – sealers and enhancers are not made to be stripped and re-coated.
Color enhancers offer the option of the stone maintaining its darker color after fading from wear. As the traffic areas dull, a good deep cleaning and application will bring it back.
Now, just because someone tells you that you are dealing with slate doesn’t mean it truly is a slate.
There are several quartzites sold as slates (and even as sandstones). Anytime someone sees a cleft on a surface, they assume it’s slate.
I have also been told – falsely – that all slates are acid-sensitive. Let me clarify: Some slates are acid sensitive. I have worked on slate floors where two of the five different colors on the floor were sensitive to acidic cleaners, where the other three were not. When you’re looking to remove considerable grout haze, testing all the varying tiles is a wise move. 
On the same note, we have one pool surround that gets a lot of hard water buildup all-around every year. Because it’s quartzite being sold as slate, none of it is acid-sensitive; an acidic cleaner makes short work of the cleaning.
I like slate, but – to be honest – I prefer mine on a pool table. Until next month, keep your stick on the ice.

¹ Clay, the word, has two distinct meanings: Clay minerals are complex silicates, the products of chemical weathering of feldspar and other silicate minerals. Clay minerals have a sheet-like molecular structure, the same structure as mica. Clay mineral sheets are carried in suspension by water and deposition from suspension when turbidity ends. Clay is also a size term defined as particles less than 1/256 mm or 0.00016" in size. Particles can also be ground-up mineral crystals, rock fragments, and colloidal lithic material, as well as clay minerals. (MIA DSDM Ver. VII, pg 10-7)