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‘Cyllell naddu’ (slate trimming knife) used for the hand-trimming of roofing slates. The knife was made for Trevor Williams (who worked at Maenofferen Quarry, Blaenau Ffestiniog, between circa 1935 – 1960) by one of the blacksmiths at Maenofferen Quarry, and is made of iron, with one edge sharpened (like a knife blade) in order to trim the slate.

The trimming knife would be used in conjunction with a ‘trafael’ (a wooden bench with a fixed iron blade). The quarryman would trim the slate to size by resting it on the iron blade of the ‘trafael’, and cutting with the trimming knife. Two sides are cut straight first to achieve a straight edge. The slate is then measured using a ‘pric mesur’ (measuring stick - a piece of wood, a nail at one end, with graduated inch steps till fourteen inches is reached, then changing to two inch steps until twenty four inches is reached). After marking the slate, the two remaining sides are then chipped resulting in a perfect rectangular slate of the proper dimensions. This process produced very fine slate waste.