The following is an excerpt from an indepth article on Genuine and Synthetic Ultramarine, written by Paul Robinson, Artist and Painting & Technical Advisor at Winsor & Newton.
Extracting Genuine Ultramarine Pigment
For the best final pigment; the best quality rock must be used. All except the highest quality of mineral produce only a pale greyish blue powder.
Once the rock is selected, the method described by Cennini is the only effective way to extract the pigment:
The rock is firstly broken down into pieces no larger than a 5 pence piece (measurement) . The crushed rock is then finely ground with water, on a mill or in a pot.
This has to be ground finely enough to pass through a 400’s mesh sieve. This is then allowed to dry, before it is passed through a 200’s mesh sieve giving powder that is ready for extraction.
Ground lapis lazuli ready for extraction
For extraction, Venice Turps, Mastic Gum and Beeswax Pellets are weighed out into a stainless steel pan and mixed whilst the pan is on a hot plate, until mixture is fully combined and of a smooth consistency.
The mixture is then transferred into another pan through a cup sieve to remove any remaining larger lumps. This pan is kept on a set of scales so the final resin weight can be known.
The lapis powder is then added to medium mix (equal parts) and stirred in until a putty like lump forms. At this stage the resultant mass is scrapped out of the pan using a wooden spoon and a pallet knife, into an enamel bowl of water at room temperature.
With hands and fingers coated in a little refined linseed oil the lump is massaged, constantly immersing it into the water. The linseed oil being re-applied to hands when necessary.
Lapis lazuli powder and medium
The lazurite contained in the rock is hydrophilic, meaning its molecules are typically charge-polarized and capable of hydrogen bonding, enabling it to dissolve more readily in water than in oil or other hydrophobic solvents.
The whole principle of the refining process is based on these water seeking properties of lazurite. The mixture of rough lapis lazuli and its ever present iron pyrites and granite is dispersed in the thick resinous putty. Massaging this putty under water combined with the hydrophilic property of the lazurite causes the blue particles to pass into the water; whereas the particles of colourless granite together with the iron pyrites adhere to the resin within the putty.
After approximately 2 hours of massaging, a fine blue powder should start to bleed out of the lump. The process is continued, swapping for a fresh bowl of water approximately every 30 minutes.The contents of each bowl are poured through a 400’s mesh before allowing to settle.
This process is repeated until no more blue bleeds from the lump.
The blue powder in the bowls is allowed to settle overnight, when any clear water is tipped off and the powder allowed to dry slowly.
The finished powder is added to boiling water to remove any medium residue, and the powder allowed to dry again.
Final Genuine Ultramarine Pigment
Genuine Ultramarine has always been an expensive colour to produce because of the very high quality of stone required, because of the very high losses incurred in refining, and because of the long and tedious method of extraction. Ultramarine was the supreme blue in medieval times and also, possibly because of its intrinsic value, the use of this pigment in conjunction with gold was popular as an expression of wealth and elegance with patrons of the Renaissance.