By Jim Hingst
When someone mentions fresco painting, what
always comes to my mind is Michelangelo painting in the Sistine Chapel on wet
plaster. Of course, that’s not the only type of fresco painting, but it is
arguably the purest and the most durable form.
Sistine Chapel ceiling by Michelangelo.
Michelangelo
[CC BY-SA 3.0
(https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia
Commons. Photograph by Antoine Taveneaux Taken on 14 June 2014.
Frescos painted in this manner two thousand
year ago still retain their color. The photo below depicts a fresco from the
ancient city of Pompeii, which was buried under volcanic ash in 79 AD during
the eruption of Mount Vesuvius .
Painting on wet plaster is known as buon
fresco. Buon means true or genuine and fresco means wet. In other words, buon
fresco means true fresco. What makes
this form of painting so durable is that the pigment is absorbed into the
plaster. Another type of fresco painting
is called fresco secco (dry), in which artist paints on dry plaster with
pigment in lime water.
Fresco showing fruit bowl, jar of wine, jar
of raisins, from the House of Julia Felix in Pompeii, Naples National
Archaeological Museum.
(This
work is in the public domain in its country of origin and other countries and
areas where the copyright term is the author's life plus 100 years or less.)
What is so fascinating to me is the chemistry
of buon fresco painting. In this unique process calcium carbonate (CaCO3) in
the form of limestone is heated to 1652°F or 900°C. In this process, which is
known as calcination, the calcium carbonate is converted to calcium oxide (CaO)
called quicklime and carbon dioxide (CO2).
The term calcination comes from the Latin
word “calcinare”, which means to burn lime. In spite of that meaning, the
process of calcining is also used in the creation of pigments. For example, when raw
umber is subjected to calcination, it becomes burnt umber.
CaCO3 + heat → CaO + CO2.
The calcium oxide is highly alkaline, which
makes it very corrosive. It is so corrosive that it can actually cause a
chemical burn on your skin. Next, water and calcium oxide are mixed to produce
calcium hydroxide (Ca(OH)2), which is called slake lime. This chemical reaction releases a tremendous
amount of heat.
CaO + H2O → Ca(OH)2 +
heat.
The slake lime is mixed with sand to form the
plaster. Instead of using sand, Michelangelo did things a little differently. He used volcanic ash, called "pozzolana", in the mixture. The pozzolana reacts with
the calcium hydroxide to form a harder plaster.
NOTE: What is also interesting is that the Romans also mixed volcanic ash with quicklime and water to create
what we today call “super concrete”, which outlasts modern concrete. As the
mixture cures, crystals form, filling any microscopic spaces and reinforcing
its structure.
The artist first pounces the pattern of the preparatory drawing, called the “cartoon”, onto the wet plaster, using the same technique that the old sign
painters used. The artist then paints on the plaster using a simple mixture of
water and ground pigment.
Michelangelo used a very limited palette of approximately seven colors. These were primarily earth colors, such as carbon black, ochres, sienna and terre verte. For blue, he used lapis lazuli. Because the plaster is highly alkaline, not all pigments are compatible with it and will fade in time.
The wet plaster dries in about six to eight hours, depending on the ambient temperature and humidity. That doesn’t give the artist much working time. For this reason, the artist only applies enough plaster that he is capable of painting in one day. The term that is used for this day’s work is “giornata”.
Michelangelo used a very limited palette of approximately seven colors. These were primarily earth colors, such as carbon black, ochres, sienna and terre verte. For blue, he used lapis lazuli. Because the plaster is highly alkaline, not all pigments are compatible with it and will fade in time.
The wet plaster dries in about six to eight hours, depending on the ambient temperature and humidity. That doesn’t give the artist much working time. For this reason, the artist only applies enough plaster that he is capable of painting in one day. The term that is used for this day’s work is “giornata”.
The wet plaster combines with the carbon
dioxide in the air as it cools. As the plaster cures, the water evaporates. What remains
is calcium carbonate, the same material that started this procedure. The
chemical processes have come full circle.
Ca(OH)2 + CO2 → CaCO3 +
H2O
The paintings of Michelangelo have endured
for more than 500 years because the pigment is encased between the molecules of
calcium carbonate of the plaster.
Do You Have Questions or Comments?
Do You Have Questions or Comments?
RECOMMENDED READING
Technique of Fresco Painting Paperback – January, 2000
by
Instructions For The Practice Of Fresco Painting (1843) Paperback by
Fresco Painting - Modern Methods and Techniques for Painting in Fresco and Secco
by
About Jim Hingst: Sign business authority on vehicle wraps, vinyl graphics, screen printing, marketing, sales, gold leaf, woodcarving and painting.
After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.
Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer. Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.
Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for publications, such as Signs Canada, SignCraft, Signs of the Times, Screen Printing, Sign and Digital Graphics and Sign Builder Illustrated. He also posted more than 500 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published. Vinyl Sign Techniques is available at sign supply distributors and at Amazon.
