These are the references I used during my iris experiments. I tried to recreate medieval paint an pigment recipes using the iris from my garden. Those experiments can be found here and the paint results with them here.
You may note that they call them 'blue lilies' with astonishing regularity. This will make noticing other references easier.
A 14th century anonymous treaty De Arte Illuminandi pg 6-7
"(Green) may be seen... in blue lilies, which are called iris, and yet are changed into a very pure green color by treatment.
The color is made from these lilies as follows. Take these fresh flowers in the springtime when they are blooming and pound them in a marble or bronze mortar and squeeze the juice with a cloth into a glazed porringer. And in this juice soak other linen cloths, clean and soaked, once or twice, in a solution of rock alum and dried. And when the cloths are thoroughly saturated with the juice of the lilies in this way, let them dry in the shade, and keep them between the leaves of books; for a very lovely green, suitable on parchment is made out of this juice, made in this way by combining it with gallarino. And note that after the cloths are dry, if they are again soaked in this juice and dried, they will be better." pg. 6-7
Daniel V. Thompson's The Materials and Techniques of Medieval Painting
"The chief rival of sap green in late medieval manuscript pianting was iris green. This was made from the juice of iris flowers, sometimes mixed with alum and thickened, like sap green, but more often prepared as a clothlet. Bits of cloth were dipped into the juice of iris flowers and dried, again and again, until they contained a sufficient quantity of the color." pg. 171
Mary Merrifield's Medieval and Renaissance Treatises on the Arts of Painting
"Lily or Iris green (verde giglio). --This pigment was sometimes prepared for use by dipping piece os linen (pezzette) into the juice and then preserving them dry." pg. ccxix
|Mary Merrifield's Original Treatises pg. 422|
|Mary Merrifield's Original Treatises pg.504|
|Mary Merrifield's Original Treatises pg. 658 Paduan Manuscript De' Colori In General|
|Mary Merrifield pg 678 Paduan Manuscript Colori Diversi|
|Mary Merrifield pg 684 Paduan Manuscript Colori Diversi|
And Mappae Clavicula has a recipe for blue from iris: