Fresco Recipe

Here’s the kitchen: our fresco shop at Skowhegan in Maine.

And here are the basic ingredients of a true fresco: slaked lime putty, washed riverbed sand, earth pigment, water.

*Time could be on that list, too.

*Lime putty is made from limestone, a sedimentary rock that’s formed through the accumulation and compression of organic matter like the skeletons of sea urchins and corals over the course of thousands of years. Its chemical composition is calcium carbonate (CaCO3).

Crush limestone into small rocks and heat it to around 1,100 degrees Celsius in a limekiln of one kind or another, and you’ll get calcium oxide (CaO), also known as quicklime.

Now—wearing a full set of personal protection equipment—slake the lime by adding water. Slaking is an exothermic process that generates a great deal of heat. Through slaking, Calcium Oxide is converted into Calcium Hydroxide (Ca(OH)2): lime putty. This chemical conversion takes some time. Lime putty fit for fresco needs to sit for at least two years. An older, more mature putty will yield a stronger plaster.

Once it’s properly aged you should plop it into a mixing bath. This lime will serve as the binder for the plaster.

Use a hoe to soften it in a mixing bath and then gradually add sand to form a sturdy and malleable plaster.

*A tip: You might also add goat hair, horse hair, or straw to your base coats for added tensile strength.

To test your plaster, scoop up a palm’s worth of the material. When packed between cupped hands it should hold the form of a loose ball. If plopped back into the mixing bath, this ball should flop and flatten without splattering.

Once you have your plaster, you can apply it to a substrate (a furred-out wall prepared with wooden or metal lath, an adobe wall, unglazed ceramic or brick…) using a hawk and a trowel.

Then, float the plaster using a float—a simple tool made of rubber or wood that gently coaxes the peaks of the plaster into the valleys, leveling the layer and lifting sand to the surface to form the toothy texture.

Build up at least three successive layers of plaster: first, the scratch coat—a rough, toothy, absorbent coat that often includes brick dust, laid directly onto the subtrate.

Then, the arriccio (from the Italian riccio, or hedgehog)—a prickly coat, made with coarse sand.

Finally the intonaco (from the Latin tunica, tunic, or covering)—a smooth, thin coat, like a delicate garment draped on the body of the wall, made with very fine sand. The intonaco is the painted layer—the layer that will receive pigment.

While most paint is made of pigment and some kind of binder (oil, egg, acrylic polymer emulsion, gum arabic…), the “paint” for a fresco is simply pigment and water: a binderless pigment suspension.

To make a suspension, form a mound of pigment with a well in the center, and slowly pour water into the wall. Use a putty knife to gently coax the pigment to drink up the water. If you’ve ever made fresh pasta from scratch, this will feel familiar!

To evenly disperse the pigment in water, use a heavy glass muller. Gently guide the pigment around a glass palette in circles or in figure-8s for twenty minutes, thirty minutes, or maybe even an hour for some particularly gritty pigments.

Over time, the crackling sound of sharp-edged pigment on glass will quiet down to a: “Shhhhhh . . . Shhhhhh . . . Shhhhh”.

Once mulled, store the hydrated pigments in jars. From this, you can draw a few spoons and add varying amounts of water. More water will yield a more transparent pigment suspension; less water will yield a more opaque pigment suspension.

The binder for a fresco painting is the lime of the plaster. When we apply a brushstroke, water—sucked in by the plaster—brings pigment with it. As water evaporates, it draws up the calcium hydroxide which meets with atmospheric carbon dioxide and crystallizes on the surface.

Here the painted mark isn’t clinging to the plaster surface…Instead, the plaster is reaching out to cling to the painted mark.

As it hardens, the plaster is converted back to calcium carbonate, the chemical composition of limestone, and the painted mark becomes a part of the wall. So this is the lime cycle. From rigid stone, to brittle powder, to flexible putty….and back again to stone, in the form of a painted fresco. Once the surface crystallizes, it closes, and is no longer receptive to pigment. So, this lime cycle introduces a time constraint.

For true buon fresco, you’ll have to paint into the plaster while it’s still fresh and receptive to pigment, before the surface closes. So, if you have in mind to paint a complex fresco at a large scale, you’ll rely on the giornata: an Italian term that translates roughly to “a day’s work,” used to describe the delineated area of plaster that can realistically be painted in a single day. (The giornata are famously evident in Giotto’s Lamentation in the Scrovegni Chapel, where lapis lazuli has faded away to reveal the boundaries where one day of work ends and the next begins.)

If I were to take this family photograph and turn it into a fresco, here’s how I would break the image down into a series of giornate.

1. I’d go top to bottom and (as a righthanded person) left to right to avoid lime-splatters,
   pigment-drips, and plaster-abrasions.
2. I’d lay out lots of plaster for simpler days.
3. And far less plaster on the days that require the most care.
4. Smaller days with far more focus for heads.
5. Because once it’s closed, it’s closed.

All images courtesy Mariel Capanna. Illustrations by Charlotte Fleming.