The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, [1862], at sacred-texts.com
It was in the time of Llyr of Defective Speech that the way of burning the cyrvens with an iron stamp was understood, that is, there was an iron for every letter, heated red hot, with which they burnt on an ebill or a board what was required; and sometimes they formed letters on wood with the small prickings of a hot fork.