Balas coloured GLASS is made thus: Put into a pot crystal frit, thrice washed in water; tinge this with manganese prepared into a clear purple: to this add alumen cativum sifted fine in small quantities, and at several times: this will make the glass grow yellowish, and a little reddish, but not blackish, and always dissipates the manganese. The last time you add manganese, give no more of the alumen cativum, unless the colour be too full. Thus will the glass be exactly of the colour of the balas-ruby.