Buckthorn Yellow from green buckthorn berries. a Paduan MS recipe.

Most students of the history of medieval and renaissance paint are familiar with Sap Green, often produced from ripe buckthorn berries. Fewer have experimented with a yellow pigment that can be made using the unripe green buckthorn berries. In Mary Merrifield's book Original Treatises a recipe from the Paduan Manuscript (662) explains how it's done!  Merrifield thinks that the Paduan Manuscript dates from the late 16th or more probably the mid or late 17th century (643).

"29. To make a good yellow for writing and miniature painting--Take what quantity you please of the berries of buckthorn while they are still green and unripe, bruise them coarsely in a mortar, put them into a vase with enough ley to cover them, and place them to boil over a slow fire until half of the ley is consumed; then strain through a cloth, and put the strained liquor again over the fire, with a little rock alum, not heating it to the boiling point, and when you have done this, put it into shells to dry, and preserve it will covered from the air."

SAFETY FIRST!  Lye is a strong base, alum an acid.  Neither should be inhaled or touched without gloves.  Wear chemical and heat resistant gloves, clothing and eye protection around these experiments and always a mask.  Perform in a well ventilated area and never breathe dust or vapors from the experiment or chemicals.  Always add powder of either to liquid not vice-versa.

Ley is defined in the Glossary before Mary Merrifield's Book. Ian the Green says, "According to the glossary in the beginning of Mrs Mary P Merrifield volumes yes, they are the same thing."  Not having medieval correct lye (from soaking hardwood ashes in water purportedly), I relied on commercially available potash (K2CO3).  Conversations with Randy Asplund, Geffrei Maudeleyne and Ian the Green all confirm this identification.

Alum is Aluminum Sulfate from Dharma Trading Company. ***UPDATE: This is the 'wrong' alum.  Rock alum should be potassium aluminum sulfate.***

I took my mostly green berries,

crushed them coarsely in a mortar with a pestle then added them to a fairly weak solution of potassium carbonate (2.51g K2CO3 + 100g distilled water) which bubbled on contact with the crushed berries.

Yellow residue on the plastic knife I used as a mixing aid.

Initially I tried a water bath to reduce the liquid gently.  Since there was very little loss after 20min I took the glass jar out and set it directly over the burner, a simmer burner, on low until the lye was about half consumed.

Strained this mixture through a linen cloth to yield 69g of juice.
I added 2.00g aluminum sulfate, which Dharma Trading company calls alum, to this juice.  The alum caused it to foam and turn brown. I let the foam settle for about 45min then added liquid to two shells and a glass jar. 
 

Foamy brown liquid after adding alum.

Here is Alexandre Saint Pierre's calligraphy with the resulting yellow ink.  Eventually I'll also publish photocopies of my notes too.