The preparation for a Fresco takes years before any paint hits the wall. One major component in this lengthy preparation is the plaster. This plaster, which is composed mainly of lime, has been used since ancient times in art and architecture.
Limestone is a carbonate sedimentary rock – which means it is a rock that is formed when a bunch of material is just left sitting there, as opposed to formed by heat or pressure (metamorphic), or fire (igneous). Limestone is most often composed of the skeletal fragments of marine organisms like coral and mollusks. Its major minerals are calcite and aragonite, which are crystal forms of Calcium Carbonate – CaCO3.
Uses of Lime
Human beings have used quicklime to build some of the greatest works of architecture – the Great Pyramids of Egypt, the Mayan Ruins of Central America, the Great Wall of China, and the ancient architecture of Rome. Even the Biltmore Estate in Asheville has a limestone facade, specifically shipped from Hallowell Quarry in Indiana via 297 private train cars, weighing over 10 millions pounds.
Limestone gravel used as a PH buffering agent. Quicklime is used in water treatment of both potable and waste water to raise the PH, as well as in agriculture to provide nutrients for plants and to neutralize the PH of soil so plants can absorb more nutrients.
In the 1800s marble sized pieces of lime were burnt to create bright light for use in military exercises, lighthouses, and to illuminate stages for theatre performances (the word ‘limelight’ comes from this practice).
About 10% of sedimentary rocks are limestone, and due to its solubility in water and weak acid solutions, most caves are created out of limestone. Groundwater has the ability to eat away at the limestone, unlike other rock structures, and this forms caves. The earliest frescoes are found in cave’s (cave paintings) and were made when natural pigments were painted onto damp cave walls. They have remained in those protected structures for thousands of years.
You know what’s interesting about the cave paintings? They are the first instances of art for art’s sake, or art for God’s sake. People weren’t living in the caves, and they were dark and hidden, so they weren’t doing them for other people to see. Scientists have figured out that the prehistoric handprint paintings found in caves around the work – Argentina, France, Indonesia – were done over generations as people made pilgrimages to these sites that must have been sacred to their cultures.
Making Plaster Out of Lime
The plaster preparation begins with burning limestone in a kiln.
Calcium carbonate is a chemical compound with the formula CaCO₃. It is a common substance found in rocks as the minerals calcite and aragonite and is the main component of pearls and the shells of marine organisms, snails, and eggs.
The thermal decomposition of limestone results in Calcium Oxide (Ca0) – a white crystalline substance known as quicklime. In the kiln, which reaches over 2000 degree F, the heat causes Calcium carbonte (CaCo3) to have carbon instability, and two of the carbon molecules to detach to form carbon dioxide (CO2), leaving the quicklime (CaO). It forms ‘quickly’ from limestone into quicklime when heated.
Quicklime is not stable, and it will eventually change back to its former hard structure if it remains exposed to air – CaO (quicklime) + CO2 (air) -> CaCO3 (calcium carbonate used for mortar, or limestone).
That to me is fascinating – I don’t know of another substance that you can completely destroy by heat, then expose to air and it will eventually turn back into its original form.
However, if you mix quicklime (Cao) with water (H20), it becomes a putty that is perfect for mortar or plastering called slaked lime (Ca(OH)2). Slaked lime ( Ca(OH)2), or calcium hydroxide, has many other names including hydrated lime, caustic lime, builder’s lime, slack lime, cal or pickling lime.
This process of slaking (adding water) causes the stored heat from the kiln to be released from the carbon molecules. This in turn produces heat, a lot of heat. The temperature can reach over 300 degrees. Therefore, the technique of slaking lime takes a lot of care, and must be executed slowly and carefully. The lime must be mixed gently with a stream of water in a slaking box. Hot piles can form and burn, so the mixture must be thoroughly wet with no undissolved remnants remaining.
High calcium lime, although dangerous to handle, is very important for the strength and durability of the fresco. Plaster not containing at least 94% lime can cause a pitted surface or walls that fall apart. Safety gear must be used, including goggles and a mask, and no skin can be exposed as the quicklime is caustic and dangerous. A proper container to store the slaked lime is necessary as well, such as underground pits below the frostline so it doesn’t freeze. Although this lime putty that is made can be used in three weeks, its best to let is soak for many years, as the longer it slakes the more elastic it becomes. Two years is a general rule, although the lime plaster for the Haywood Street Fresco was slaked for 15 years.
There is something about the limestone that makes the plaster: how it purifies water, how it is the only material that can be burned, then water added to it and turn back into itself, made more beautiful with pigment. Something about the heat that when you add water (H20) to quicklime (CaO) Calcium Oxide.
The preparation for a Fresco is not as simple as gessoing a canvas or cleaning and priming a blank wall. The wall for a fresco must be stable enough to hold the multiple layers of plaster that go into the work, beginning with the hefty scratch coat. The thick scratch coat provides a solid base for the fresco, the quality and thickness used to prevent cracking and extend the painting time and strength of the final coat (the intonaco).
It is said that fresco is ‘painting with molten marble’. Chemically, limestone and marble are the same. In geology marble refers to metamorphosed limestone – or limestone (calcium carbonate fossils) that have been changed by heat.
The quality of the lime used in the fresco, from scratch coat to arriccio to intonaco, is important, not only for the strength and longevity of the piece, but also the tonality. Although lime appears opaque when freshly painted, with time the lime will continue to carbonize, slowly bringing more depth and richness to the fresco due to the ability of lime to reflect light and color. The lime will also grow in transparency (as opposed to a glue or oil bonding agent), growing the depth and luster in the lights. This adds to the list of skills the fresco painter must have – knowing the reaction of the color and lime that will change over weeks and months, and preparing the painting accordingly – essentially painting almost blind.
What if you just paint on dry limestone/plaster?
Painting on dry plaster is called Fresco secco. This is a superficial painting method – where the pigment is mixed with a bonding agent such as tempura or glue, and lays on top of the surface it is painted, therefore is less durable than true Buon fresco, where the pigment has mixed with the plaster and become part of the wall. Fresco secco is often used for touch-ups on Buon fresco’s, but the pigments will eventually fade or flake off.
The Haywood Street Fresco:
Fres·co /ˈfreskō/ noun: fresco; plural noun: frescoes; plural noun: frescos
- 1.a painting done rapidly in watercolor on wet plaster on a wall or ceiling, so that the colors penetrate the plaster and become fixed as it dries.
Haywood Street Timeline
10/25/2018 wall measured out
10/30/2018 – cutting into wall, steel framing started.
12/7/2018 – plaster.
12/13/2018 – scratch coat applied onto a metal lathe.
07/01/2019 – sinopia
07/09/2019 – fresco begins…
Visit: www.haywoodstreetfresco.org for more information.
More Resources on Lime and Fresco