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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.

Cyllell naddu a ddefnyddir ar gyfer naddu llechi â llaw. Mae'r gyllell wedi'i gwneud o haearn, gydag un ymyl wedi'i hogi (fel llafn cyllell) er mwyn naddu’r llechen. Byddai'r gyllell naddu yn cael ei defnyddio ar y cyd â thrafael (mainc bren gyda llafn haearn sefydlog). Byddai'r chwarelwr yn naddu'r llechen i'w maint drwy ei orffwys ar lafn haearn y drafael, a thorri gyda'r gyllell naddu. Mae dwy ochr yn cael eu torri'n syth yn gyntaf i sicrhau ymyl syth. Yna mesurir y llechen gan ddefnyddio pric mesur (darn o bren gyda hoelen ar ei flaen a stepiau modfedd a dwy fodfedd ar ei hyd - stepiau modfedd hyd bedair modfedd ar ddeg, ac yna bob dwy fodfedd nes cyrraedd pedair fodfedd ar hugain). Ar ôl marcio'r llechen, caiff y ddwy ochr sy'n weddill eu naddu gan gynhyrchu llechen betryal berffaith o'r maint priodol. Cynhyrchodd y broses hon wastraff llechi man iawn. Perchennog y gyllell naddu hon oedd Trevor Williams a weithiodd yn Chwarel Maenofferen, Blaenau Ffestiniog, rhwng 1935 - 1960. Cafodd y gyllell naddu ei gwneud I Trevor Williams gan un o ofaint Chwarel Maenofferen.