The article does say, "All these papers, and nearly all that Morris then went on to design were printed using hand-cut woodblocks loaded with natural, mineral-based dyes." Is arsenic classed as a natural, mineral-based dye?
I'd say yes: Arsenic is present in dyes classified as natural, mineral-based, according to https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315675/ ("Natural Colorants: Historical, Processing and Sustainable Prospects").
