Hello slate people
How would you check the state of a slate shingle? Ring it. The sound given by the slate when you hit it softly with a hard object, like a hammer or a rubber mallet, tells more about the structural integrity of the shingle than any laboratory test (and it is cheaper). This is not new news, everyone with a bit of experience on roofing slate already knows it. Ringing the slate is the best way to predict its future performance. This is especially useful when you are reclaiming slate, and need to separate the good ones from the damaged. Easy and simple. If the pitch is sharp and clear, the slate is perfect, but if the sound is dull and fuzzy, then you know that the slate must be discarded. But how this works, which is the scientific principle that supports this method?
Whenever two objects strike, there is a transmission of energy from the object with more energy to the other. This energy is in form of kinetic energy, or movement energy. For example, when a cannon ball strikes your head, the object with most energy (cannonball) transmits it to the object with lower energy (your head. The speed of the cannonball is the amount of energy that it has, and determines the final effect on your head. This experiment has been performed many times before, so no need to do it again. Getting back to the slate, when the hammer strikes the slate shingle is transferring it energy. This energy goes along the shingle, creating a vibration. If the slate is homogeneous and perfect, without any imperfection (cracks, quartz veins, pyrites…anything but slate), the slate mass will vibrate at the same time, producing a single sound wave. The result is this crystalline sound, like hitting a wine glass with a fork. On the other hand, when the energy wave finds discontinuities caused by the aforementioned cracks and internal heterogeneities, the slate mass vibrates at different wave lengths, being the resulting sound a combination of all. The sound is then unclear and soft.
Many times people have asked to me which are the best tests for determining roofing slate quality. Well, you can perform expensive test that will give you (after several weeks) information about the chemical and mineralogical composition of the slate, together with the mechanic properties and the vulnerability to weathering. These tests are important and any quarry must have performed them on their products, but for everyday work, when you need instant information about a slate batch, other methods are needed. It comes to my mind something I heard this summer, when talking with a retired slate worker in Spain. He told me that back in the 80´s, the German market was considered the most demanding market. They wanted only first quality, perfect slate. Very picky people, they paid well for quality, so making business with the Germans was a sign of excellence. One of the things the Germans demanded was ringing every single slate before putting them into the crate. This increased the price a bit more, so better business for the Spanish…or not? You can expect about 2% of breaks on a first quality crate. If the slates are already ringed, the broken ones are already discarded, so when you open the crate you can expect that 100% of your slates are perfect, no losses due to breakages. This represents a gain higher than the price increase for ringing all the slates. Clever lads these Germans.