When ordering natural cork granules (as opposed to expanded cork granules), density is the most important factor that you will have to consider. Cork granule density can vary between 60 kg/m3 and 220 kg/m3 and the properties and appearance of the granules along this spectrum will vary enormously. In this article we will look at what density of cork granules are available for different usages, but first an explanation of why cork granule densities can vary so much.

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Cork is produced when the bark is stripped off cork oak trees (incidentally, this does not kill the tree and the bark will regenerate in around 9 years when it can be harvested again, making cork one of the most environmentally friendly products on the planet!). This cork bark will have a soft inside layer (where it is nearest the trunk), but the outside of the bark will be hard, brittle and woody. This bark is cut into strips and natural corks are punched out of the softer layer, leaving behind off-cuts which will include both softer cork from in between the punch holes and hard cork from the outside bark.

Cork being punched out of bark strips. Note the darker colour outside of the bark at the top of the photo.
Here you can see the punched-out cork bark strips, ready for the grinder to make cork granules

These off-cuts are then passed through a grinder , producing granules of various sizes and densities. These granules are then sorted by size (using mesh screens) and densities (using sloped shaking tables) to form a range of options in various bands of granules size and density. So for example we might supply you with cork granules of 1-2mm and 120-140 kg/m3.

The cork granules are vibrated on a sloping surface to separate them by density.

The lower density granules from around 60 to 80 kg/m3 are the most prized as they are the most elastic and are mainly used to make micro-agglomerated wine cork stoppers and micro-agglomerated bar-top cork stoppers as well as blocks of agglomerated cork which can be used to make cork lids and other agglomerated cork items. Higher density granules cannot be used to make cork stoppers, because they are insufficiently elastic to make a good seal in the bottle, but there are a lot of uses for higher density granules.

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The largest use for mid-density granules (so 90 kg/m3 up to 180 kg/m3) is cork flooring, but they are also used increasingly for artificial turf cork infill as well as for coarse granule insulation boards and a range of other usages. As the density gets higher, so the cork granules become more brittle and less elastic, so the cork begins to behave more like wood. Because there is less demand for cork granules the higher the density is, the price also decreases dramatically with density. So for the lowest density cork granules you can pay over €4 per kg, but for the highest density granules, less than €1 per kg.

The size of cork granule does not generally effect price and will be produced in bands, starting at 0.2-0.5mm and going up to 8-12mm. Smaller than this granule size is cork powder or dust, which is produced when cork products are ground down to size using grinding stones, which makes an extremely fine powder of around 200 nanometres in diameter.