Cutting slate

Cutting Slate

Cutting slate is a skill still in demand. Slate is a fine grained layered rock, so cutting slate after its been mined in large blocks is a particular skill. But for the geologically minded interested in cutting slate, it may be of interest to know what slate actually is. Slate is an example of metamorphic rock originating in ‘shale like’ sedimentary deposits originally composed of volcanic ash and clay. Geological factors creating heavy compression have caused the layering to develop in what is called ‘slaty cleavage’ causing the grained flakes of clay to rise perpendicular to the compression. Cutting slate expertly however delivers, almost miraculously to the untrained eye, flat smooth sheets of stone which have been used in roofing for centuries. Who discovered the art of cutting slate is a mystery. Although 90% of Europe’s slates used for roofing come from Spain, our own North Wales has made a noble contribution. Cutting slate usually involves cutting the ubiquitous grey rock but slate from North Wales can be found in a number of variations of the standard grey as well as in green, purple or even greenish blue variations of cyan. Best cutting slate is composed mostly of illite, muscovite and quartz with a variety of other minerals thrown in. And occasionally however some North Wales purple coloured slates exhibit spheres which have formed around nuclei of iron, creating a light green spotted effect, which following application of stress, look like ovoids.

hand dressing slates

hand dressing slates

 

Following extraction by mining the next step is cutting slate, usually into roofing tiles, and that’s where the skill of cutting slate comes in. The rock has two lines where it can fracture, the grain and the cleavage, which make cutting slate into perfect flat tiles achievable and even after cutting, slates keep their natural appearance whilst nevertheless functioning like an artificially manufactured product. After cutting, slates are stackable and stay flat. Following cutting, slate is especially valuable as roofing material and even following cutting, slate has a very low moisture absorption capacity making it immune from the effects of freezing weather. Slate is fireproof and is a good electrical insulator so the art of cutting slate found what would then have been a high tech role, at the start of the 20th Century when demand for skills in cutting slate found a place in making relay controls for electric motors and the manufacture of electrical switchboards. Owing to its chemical inertness and thermal stability slate has been useful for making laboratory work surfaces.

guillotine for slate cutting

guillotine for slate cutting

Cutting Slates into Tiles

Cutting slate into tiles of varying styles makes it suitable for use as flooring, for kitchen surfaces, roofing tiles, wall cladding and for laying on walkways and stairs. Following cutting, slate tiles can be set on mortar beds and grouted like ceramic floor tiles. Forroofing they are fixed with nails onto timber battens. Chemicals sealants are now used on sometiles after cutting. Slates can sometimes suffer from a crystalline deposit forming on the surface caused by ‘efflorescence’ (leaching out of mineral rich hydration from the slate). Efflorescence can, if the slate is not treated, continue over an extended period after cutting. Slates chemically treated to avoid this phenomenon are more stain resistant, and can be made to order in bespoke degrees of surface smoothness. After cutting, slates are often ‘gouged’, meaning that following cutting, slates have had their back surfaces ground to make them easier to lay.

Drill attachment for slate cutting

Drill attachment for slate cutting

Even without cutting, slate is usable in its natural form. Visitors to some villages in North Wales could be forgiven for thinking that the whole landscape, including the houses, is carved out of slate. Without any need for skilled cutting, slate was used to build houses. Elsewhere, again without much need for skilled cutting, slate has been set in walls to help with damp proofing and small bits left over from cutting slate into tiles can be used as no compressive packers for raising the height of floor floor joists or pretty much anything else. In localities where slate is easy to come by slate has a variety of other uses without much cutting. Slate in its rough form and in a variety of sizes is ideal for building  walls and the left overs from cutting slate can often be turned into beautiful ornaments for homes or used as addressing for paths and driveways.

During the process of cutting, slate is ‘squared’. Then by using a traditional measuring stick immediately prior to cutting, slate is scored across the surface to obtain the biggest slate tile possible from the piece of slate   The art of cutting slate tiles has resulted in some quaint traditional names for the different standard sizes cut, ranging from variety of sizes of a style called ‘Drains’, 14x10s, Narrow Ladies, Broad Ladies, Wide Ladies, Narrow Countesses, Princesses, Duchesses, and Countesses. Following cutting, slates are ‘dressed’. That however is a subject in itself which we may cover later and is a much more achievable challenge for an amateur to tackle than actually cutting slate from a mined block.   

Cutting Slate at Home

Mercifully unless you are attempting to make something very unusual cutting slate is not something you need tackle yourself and is not necessary. Slates come pre cut and when you need a slate of a slightly different size to the standard sizes available all that is required is a few tools to trim it down. That is the extent to which we recommend amateurs venture into cutting slates. Traditionally, mason’s trowels were used for this final stage of cutting slates to fit but nowadays renting a purpose built tool designed for cutting slates is a better option. But cutting slates with a trowel is still possible. Whatever method you use always cut the tiles face down to avoid splintering.

Wet tile saw

Wet tile saw

Cutting Slates with a Electric Cutter

  1. Put  the tile on a level surface face down and prepare a tape measure to the exact width you require
  2. Put the tape measure on the tile with the tape measure casing against the side of the tile and the end of the tape resting on the tile. Slide the end of the tape down the tile scoring a line in the tile at the measurement you require
  3. Insert the tile face down into the slate cutter. Keeping the tile firmly on the bed commence cutting the tile slowly along the scored line. Insert the tile further into the slicer as you go. Continue until you’ve cut away all the unwanted tile
  4. Handle devices for cutting slates with care and keep them away from children
  5. Fix the iron into a solid piece of wood. Lay the tile on the edge of  the iron with the scored line facing upwards. The piece you wish to cut away should overhang the edge.
  6. Holding the tile firmly in place, use the trowel to chop along the scored line. The excess slate will fall away
  7. Keep the scored line flush with the side of the iron and remove any jagged edges with the trowel
  8. This method is very quick and is used by slate roofers and only if the material is thin.
Used in combination with the masons iron for slate cutting

Used in combination with the masons iron for slate cutting

Cutting Slates with a Mason’s cutter and iron

Prepare the tile in the same way as you would have when cutting slates with the Slate Cutter

  1. Fix the iron into a solid piece of wood. Lay the tile on the edge of  the iron with the scored line facing upwards. The piece you wish to cut away should overhang the edge.
  2. Holding the tile firmly in place, use the trowel to chop along the scored line. The excess slate will fall away
  3. Keep the scored line flush with the side of the iron and remove any jagged edges with the trowel. 

 

slate iron with point for fixing into timber

slate iron with point for fixing into timber