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Slate trimming knife (cyllell naddu) used for the hand-trimming of roofing slates. Used at Dinorwig Quarry between 1967-1969. The knife is made of iron, with one edge sharpened (like a knife blade) in order to trim the slate. One part of the handle is missing (the wooden hilt).

The trimming knife would be used in conjunction with a ‘trafael’ – a 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. It is then measured using a measuring stick (pric mesur) - 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.