Slaked Plaster

Carbon Inks

Quill Pens


Iron Gall Inks



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Slaked Plaster

Plaster of Paris is made up of one water molecule shared between two calcium sulfate molecules. The inert bulk formers used in gesso (chalk, gypsum, etc) have at least a 1:1 ratio, if not better. The drier Plaster of Paris will therefore absorb moisture instead of letting it build it up on the surface. Slaking adds the necessary moisture so that it can be used correctly in gesso. 

Medieval recipes say to put the plaster in a bucket, fill it with water and mix it vigorously for 30 minutes. Let it sit overnight. The next day when the plaster has settled, pour off the old water, add new and repeat the process. This would be done for a month or more. The Plaster of Paris we buy at the store, however, is fairly well processed. It would take a bit longer to slake roasted gypsum (the medieval version Plaster of Paris) because it was full of impurities. This is not the only reason we can make in a short time what it took scribes in period weeks to do. Jerry Tresser (The Techniques of Raised Gilding) claims that the long periods of time the plaster sits at the bottom of the bucket does nothing. It's the mixing and changing water that does the job. Plaster of Paris is acidic. Slaked plaster, like the distilled water used to make it, is neutral. When the reading on a piece of litmus paper is neutral (pH 7), the plaster is slaked and ready to be dried and stored.

Plaster of Paris = Calcium Sulfate Hemihydrate: The hemihydrate part of the name means that when the material crystallizes and becomes solid, only one water molecule is shared between two calcium sulfate molecules. Also called dried calcium sulfate, dried gypsum, Annalin. This form of calcium sulfate will harden when mixed with water.

Calcium Carbonate: Exists in nature as the minerals aragonite, calcite and vaterite (types of limestone); Commercial names: Precipitated calcium carbonate, precipitated chalk, Aeromatt, Albacar, and Purecal; Native names: Precipitated calcium carbonate, drop chalk, prepared chalk, whiting, English white, Paris white. Calcium carbonate is unaffected by alkalis and remains white when heated. Impure varieties discolor; when they contain iron they turn red. When strongly heated, calcium carbonate converts into quicklime. It is non-poisonous and has little covering power. The whiter it is, the greater the value; therefore French chalks are the best. 

Chalk = calcium carbonate: (see above)

Marble = calcium carbonate:  (see above) Whether one finds chalk or marble in the ground is determined by the conditions that were present when each one formed. A classic example is graphite (pencil lead) versus diamond. Both are pure carbon, but diamonds form when there is high heat and lots of pressure. Without those conditions, you get graphite. Quite a difference!

Gypsum = Calcium Sulfate Dihydrate: A mineral consisting of the hydrous sulphate of lime (calcium). When calcined, it forms plaster of Paris. The dihydrate part of the name means that when the material crystallizes and becomes solid, two water molecules are trapped with each calcium sulfate molecule. Also called native calcium sulfate, precipitated calcium sulfate, alabaster, selenite, terra alba, satinite, mineral white, satin spar, light spar. Selenite is a transparent, crystalline variety; alabaster, a fine, white, massive variety. This form of calcium sulfate will dissolve in water. 

Thanks to Lady Eibhlin ni Chaoimh and Lady Karana Yabokchi for much of this information.

Quill Pens

The barrel of a quill is made of albumen, a form of the protein similar to our fingernails and hair. Soaking the quill enlarges the cells, and tempering with hot sand or a dutching tool causes them to suddenly shrink again, forming a stronger bond in the proteins.

The tannic acid in oak gall ink has a curative effect on quill pens, much the same way that it effects leather. The essential result in tanning leather is due to the fact that the tannins form, with gelatins and albuminoids, a series of insoluble compounds. 

Tannic acid: (a) An acid obtained from nutgalls as a yellow amorphous substance, {C14H10O9}, having an astringent taste, and forming with ferric salts a bluish-black compound, which is the basis of common ink. Called also tannin and gallotannic acid.
(b) By extension, any one of a series of astringent substances resembling tannin proper, widely diffused through the vegetable kingdom, as in oak bark, willow, catechu, tea, coffee, etc.

Tannic acid is contained in the galls, bark, leaves, roots and fruits of various plants. The greatest concentration of gallotannic acid is found in galls (the bulbous growths formed on the leaves and twigs of trees in response to attack by parasites. Galls are collected from oak, oak-apple and pistachio trees. A lower proportion of gallotannic acid may be extracted from the bark of various trees, including oak, chestnut, mountain ash and cherry. Various other sources for tannin include pomegranate rinds, horse chestnuts, hemlock and pine bark.

Iron Gall Inks

Iron gall ink is essentially created by the chemical reaction between tannic acid and iron (II) sulfate in an aqueous solution. The primary active components in tannin are gallotannic and gallic acid. With iron (II) sulfate, these tannic acids produce the black pigment ferrogallotannate, or ferrotannate, upon exposure to oxygen. A small amount of pigment forms by reacting with oxygen in the water, but much more pigment is produced after the ink has been applied to paper and exposed to air for several days. Even though iron gall ink has been highly prized for centuries for its durability and rich color, it is known to be chemically unstable, and may, over time, change color or damage the paper on which it is applied. A 3:1 ratio of gallotannic acid to iron sulfate produces the most stable inks. 

Scientific literature names the following reasons for ink degradation of paper: (1.) the high acidity of some inks which contributes to the hydrolytic splitting of the cellulose; (2.)the efficacy of soluble iron compounds as catalysts for the oxidative decomposition of cellulose. For more information on gall ink corrosion, click here.

Gallotannate: Gallotannate is extracted from galls by powdering or crushing the galls and mixing with water or other liquid; by boiling whole galls for several hours to release the tannins; or by fermenting the galls by mold. The fermentation process generally produces the richest, blackest inks. As the mold enzymatically digests the gallotannic acid, the solution is transformed to gallic acid. Gallic acid will produce a purer black color in reaction with iron sulfate, while gallotannic acid will produce a comparatively browner pigment. 

Iron (II) sulfate: Iron sulfate is called by many different names, including ferrous sulfate, vitriol, and copperas. Artist manuals distinguish iron sulfate as green copperas and copper sulfate as blue copperas. Pure iron sulfate may be obtained from chemical, specialty art or fabric dye suppliers in the form of a pale green powder or granules. A less pure form may be made at home by dissolving iron scraps or nails in a weak acid. However, making your own iron sulfate should never be attempted without a good understanding of the health and safety hazards involved. 

Water or wine: Most inks are made in water. Water from the tap may be contaminated with chlorine, metals from pipes, calcium and other salts. For this reason it is generally better to use fresh rain water or distilled water instead. One milliliter is approximately equal to one gram. 

Wine, beer or vinegar were sometimes used in medieval recipes because it was thought to be a purer liquid. Alcohol may also have prevented the ink from freezing in winter, but, since some recipes require boiling the alcohol (which would cause it to evaporate), there may be another explanation for its use. It may be that the glycerin in alcohol increases the rate of extraction for tannin. Alcohol also reduces the surface tension of the ink solution, allowing it to soak more quickly into the paper fibers. Anecdotal evidence suggests that a large proportion of alcohol or vinegar may have a preservative effect, inhibiting mold from growing on the finished ink. 

Gum arabic: Gum arabic is a water soluble golden-colored sap collected from Acacia trees native to North Africa. It can be purchased from art supply stores in the form of a liquid, a powder or as dried clumps or fragments. Gum arabic suspends the black pigment in the liquid; otherwise, it would settle to the bottom of the container over time. It also helps to thicken the ink, allowing it to flow more easily from the pen or brush onto the paper. 

More importantly, the gum holds the ink at the surface of the paper for a few extra seconds before sinking into the fibers. This influences the appearance and durability of marks made with the ink. The ink line is clearer and sharper than it would be without a binding agent, in part because the ink sinks less deeply into the paper fibers. However, too much gum arabic will cause the dried ink to become inflexible, and it can crack and flake off the surface. 

For more information on making iron gall ink, see "How to Make Iron Gall Ink," by Cyntia Karnes, Museum Boijmans van Beuningen, Rotterdam.

Carbon Inks

Carbon-based inks consist of practically pure carbon in amorphous conditions, mixed in suspension with a gum binder.

Gas black: Derived from the incomplete combustion of gases and substantially free from grease.

Lamp black: Derived from the incomplete combustion of oils.

Chinese carbon-ink: Derived from the incomplete combustion of pine-boughs (or rice-straw, haricot beans, tung-oil, sesame oil, etc.) at a carefully regulated temperature. The resultant soot is collected in screens high above the fire; shaped into sticks by adding animal resin and certain herbs, steaming in earthenware vessels, and hammering into shape to guarantee an even mixture of materials. Chinese ink is prepared by rubbing the stick on an inkstone with water.


Shell Colors: It was common practice for medieval scribes to make paints in individual shells (like clam shells). Such shells are made of calcium, causing pH changes in some colors when they are being used. If you use plastic or glass, that change won't happen until the color gets to the parchment (made with lime process and therefore having a similar pH to the shell). 

Pigments: For more information on the chemical and historical properties of many medieval and contemporary pigments, see my Pigments page.


An egg is nature's perfect emulsion. Emulsions are mixtures of oily, waxy or resinous substances suspended in a more liquid medium like water. Eggs contain water, a gum like substance called albumen, fat and oil, with an emulsifier called lecithin. Exposed to air, the water in an emulsion evaporates quickly. This accounts for egg tempera paint's quick setting time. The fats and oils require more time with the aid of emulsifiers to cure completely.

Chemical Composition of an Egg:


Yolk (%) 

White (%) 

Water 49.5  86.2 
Albumen, vitellin etc.  14.5  12.7
Fat or oil  18 0.2
Lecithin etc.  11 traces
Mineral matter  1.0  0.7 
Other substances  1.5 2.3

For more information on egg tempera painting, see The Society of Tempera Painters.