In the Early Gothic, period. the glass was particularly thick and was deeply coloured with metal oxides; cobalt for blue, copper for a ruby red, iron for green, and antimony for yellow. The process of making the windows was described detail by the 12th-century monk known as Theophilus Presbyter. The glass of each colour was melted with the oxide, blown, shaped into small sheets, cracked with a hot iron into small pieces, and assembled on a large table. The details were painted onto the glass in vitreous enamel, then baked in a kiln to fuse the enamel on the glass. The pieces were fit into a framework of thin lead strips, and then put into a more solid frame or iron armatures between the panels. The finished window was set into the stone opening. Thin vertical and horizontal bars of iron, called "vergettes" or "barlotierres", were placed inside the window to reinforce the glass against the wind.