Activity n°2 "Chemistry of Art, Art of Chemistry"

Managing my activity in classroom step by step

 

 

 

 

Objectives

 

To learn about paint’s main components: binder, pigment and thinner

To discover artists’ use of binders and pigments through the ages (Prehistory, Antiquity, Medieval Age, Renaissance, Modern age, Contemporary age)

To understand the contribution of chemistry to the creation of new, high quality binders and pigments

To synthesize inorganic pigments

To carry out trials using a range of binders to prepare 5 different kinds of paints: tempera, oil paint, acrylic, pastel and watercolors

Showing these different types of paints in several works of art.

Carrying out the activity step by step

Upload the complete documentation:

1/ a PDF guide to implement the activity in your class step-by-step;

2/ a  Powerpoint presentation to be projected in front of the whole class during the activity;

3/ a complete list of chemical products and materials in Excel file.

 

INTRODUCTION: THE CHEMISTRY OF ART
Slide 2: What do cave paintings and contempory paintings have in common?

The focus of this activity is about art paintings. Can someone define what a paint is? A paint is a kind of covering applied to a surface (substrate) with two main goals: decoration and protection. In art, the paint is mainly decorative, but this does mean that it is durable, chemically and mechanically resistant. This explains why these cave paintings founds in the south-west of France have crossed the age. Art paints have made considerable progress through the ages, from cave paintings to this contempory painting of Marc Rothko, the materials, techniques are very different, and the main reason is chemistry. The main objective is to highlight the contribution of chemistry to the art paint techniques. The two paintings are very different, but they also have common paints, in the sense that the basic materials making up paints have remained unchanged for thousands years. There are still three main components:

  • Pigment – to colour (indicated in white on the slide)
  • Binder – to bind the powder pigment into a paste (animal fat for cave painting, vegetable oil for Rothko)
  • Thinner – to adjust viscosity (material texture)

 

Slide 3: paint preparation

The image shows the preparation of an oil paint. Cobalt blue (Pigment) being ground with linseed oil (Binder) and turpentine (Thinner) to make a paint.

 

Slide 4: Key components of a paint: Binder, Pigment (and Thinner)

The first short video (1:32) produced by the MOMA of New York is entitled “Arts Terms in Action: Paint”. It briefly presents the two main ingredients of paints: the “binder (linseed oil here) and the “pigment”. Thinner is not mentioned but turpentine is shown in the image. Explanation about the different components are provided.

 

Slide 5: Pigment

Solid particles in the micrometer size range. Insoluble with the other components of the paint (suspension). Coloured because they selectively reflect and absorb certain wavelenghts of visible light. Natural or synthetic. Mineral or organic.

 

Slide 6: Binder

Microscopy cross-section of paint (slice through the paint). In different microscope images, we see that paintings and other painted objects consist simply of pigments suspended in a susbstance (binder) – like chips in a chocolate chip cookie. The binder can vary, from oil or egg yolk in paintings, to more sophisticated plastics in automobile coatings. On the left, the top layer is varnish, a transparent (brown here because of oxidation) that is meant to protect the painting against weathering, UV irradiation.

 

Slide 7: Thinner

Diluting medium (solvent) to adjust viscosity and ease paint deposition. Give the example of honey that is too viscous to ensure a proper deposition on a support, and needs to be diluted. The thinner evaporates after brushing (not present in the final painting after drying). Two types are used depending on the binder: turpentine (can be replaced by petroleum ether, easily found in a chemistry lab) for oil paint and water for the water-based paintings (tempera, acrylics, watercolors). Using water to mix paint is very familiar to pupils and is known as painting in watercolors. In this case, the main components are gum Arabic (binder), water-dispersible pigments, and water as thinner. However, there are other possibilities for making paint with different binders, thinners and pigments (see activity n°4).

 

Slide 8: Content of our hands-on activity
  1. Learn about the general make up of paints by playing with cards
  2. Preparation of two pigments from antiquity: Azurite and Malachite
  3. Preparation of two pigments from modern ages: Prussian blue and Colbalt violet
  4. Prepare your own paint: Oil Paint, Tempera Paint, Acrylic Paint and Watercolors.
ACTIVITY N°1: THE GENERAL MAKE UP OF PAINTS BY PLAYING WITH CARDS

Students work in trio. Each trio receives:

– A set of 12 cards

– A serie of 12 samples including pigments, binders and thinners:

2 sheets for classification 1 (component type), 2 (natural or/and synthetic).

 

The teacher provides background about the subject then lets 4-5 min to the students for the classification of cards depending on “component type”, or “natural/synthetic character”. Explanation/correction is performed with the whole class

ACTIVITY 1.1: USING A SET OF 12 CARDS ILLUSTRATING PAINT COMPONENT, THE PUPILS IDENTIFY EACH AS BINDER, THINNER OR PIGMENT

Slide 9: Classify in terms of main paint’s components

The demonstrators let around 5 min to the students to arrange the cards in the three different rows (binder, thinner, pigment). In the meantime, the demonstrators circulate among the students to answer questions and assist them in the classification of cards. The answers are eventually projected. A discussion on each material can be started depending on the pupils’ questions. The demonstrator shares “background information” at an appropriate level with the students. We propose to focus on one particular material (see next slide).

Slide 10: Focus on linseed oil

The demonstrator cannot comment with details on all items but a particular attention can be paid to linseed oil. Linseed oil is extracted by crushing flax seed produced by flax flower. Flax is also used in the textile industry for fibers. In terms of chemical structure, linseed oil is based a triglyceride molecule, like most vegetable oils. It is liquid at ambient temperature (show the bottle), sensitive to light. It contains three ester functions = triester of fatty acids. It contains a high concentration of carbon double bonds = unsaturations (reactive sites). These chemical groups account for the ability of this binder to produces a hard, insoluble film when it dries (see slide 21 for details).

 

ACTIVITY 1.2: CLASSIFICATION IN 3 CATEGORIES: NATURAL / SYNTHETIC / NATURAL + SYNTHETIC

Slide 11: Classify in terms of natural and synthetic characteristics

The demonstrators let around 5 min to the students to arrange the cards in the 3 different rows (natural, synthetic, natural and synthetic). This latter category refers to materials that both exist naturally and can be chemically synthesized through technical development. These synthetic products are not strictly identical to the natural counterpart but are cheaper to produce. In the meantime, the demonstrators circulate among the students to answer questions and assist them in the classification of cards. The answers are eventually projected.

Slide 12: Focus on Rose Madder

Can you recognize the section of Johannes Vermeer’s paint including Rose Madder ? This is the red blouse of Mary! Rose Madder is derived from the extract of the madder plant’s root (rubia tintorum) . It is thus a Natural pigment. It was used by the ancient Egyptians for colouring textiles and then was continuously used until today. Not often used as pigment for paintings, and most often used for coloring (dyeing) textiles, in particular for the French military cloth during world war I. It is composed of three main molecules (alizarin, purpurin, pseudopurpurin, they all belong to the family of anthraquinone). Alizarine (red) is the most stable and also a synthetic pigment (chemists knows how to fabricate it).

 

ACTIVITY 1.3 (OPTIONAL): Using the same set of cards, the pupils indicate to each period of ages the materials belong to: antiquity, medieval age, renaissance, modern age, contemporary age.

 

Slide 13: Identify paint materials through ages
ACTIVITY N°2: PREPARATION OF TWO PIGMENTS FROM ANTIQUITY: AZURITE AND MALACHITE

The students work again in trio for the synthesis of two pigments. A written protocol is provided to all students. It is first shown by the teacher in front of the class, then carried out by each group of three students.

Azurite and Malachite are synthesized successively according to the reaction:

  • Azurite: Cu(CO3)Cu(CO3)Cu(OH)2
    3 CuSO4 – 5H2O(aq) + 3 Na2CO3(aq) → Cu(CO3)Cu(CO3)Cu(OH)2(s) + 3 Na2SO4(aq) + 3 CO2(g) + 14 H2O(l)
  • Malachite: Cu(CO3)Cu(OH)2(s)
    2 Cu(CO3)Cu(CO3)Cu(OH)2(s) +3 H2O(l) → 3 Cu(CO3)Cu(OH)2(s) + CO2(g)

Experimental protocol: in a 150 mL beaker 1, 5 g (0.02 mol) of CuSO4 – 5H2O (pre-weighted in plastic test tube) is dissolved in 20 mL of distilled water. In another 150 mL beaker 2, 2.1 g (0.02 mol) of Na2CO3 (pre-weighted in a plastic test tube) is dissolved in 20 mL of distilled water. Label the glassware to avoid any confusion. Ensure a dissolution of both compounds in water by gentle stirring or magnetic stirring after addition of a magnetic bar. The carbonate solution (beaker 2) is added gradually with a Pasteur pipet to the copper solution (beaker 1). The evolution of carbon dioxide gas is controlled by the addition of Na2CO3. After addition, half of the reaction is added to a third 150 mL beaker 3 for the production of malachite. The precipitate of insoluble blue coloured azurite contained in beaker 1 is filtered and dried. The beaker 3 is heated during 5-10 min under vigorous stirring, the medium becomes greenish, suggesting the transformation of azurite into malachite. The precipitate of green coloured malachite is filtered and dried. Keep all the pigments, which may be used in activity n°4

 
Slide 14-15: Experimental protocol for the synthesis of Azurite and Malachite

The demonstrator shows in front of the class the different steps to prepare the two pigments

 

Slide 16: A Focus on Azurite and Malachite pigments: when blue becomes green!

Azurite was known for antiquity. It was the most important blue pigment in European painting throughout the middle ages and Renaissance. Produced artificially from the 17th century, it was replaced when “Prussian blue” is discovered in the 18th century. Malachite is perhaps the oldest known green pigment. The first occurrence was in Egyptian tomb paintings. In European paintings, it seems to have been of importance mainly in the 15th and 16th centuries. The demonstrator shows a painting from Raphael. He points out that Mary’s mantle was originally blue because it contained azurite pigment, and ask the students why it became green over time? The azurite blue of the Virgin’s mantle has darkened due to its degradation into green malachite.

ACTIVITY N°3: PREPARATION OF TWO PIGMENTS FROM MODERN AGES: PRUSSIAN BLUE AND COLBALT VIOLET

The students work in trio for the synthesis of two new more recent pigments. The procedure is shown by the teacher in front of the class, then carried by each trio. Each group received a test sheet for the deposition of the solution, and there is a set of dropper bottle available.

For the preparation of a 10 mL solution at a concentration of 0.2 M, please weight w g of solids, w = 0.002 * Molar mass(salt). See pdf guide for details

Slide 17: Preparation of pigments from modern ages

The idea of this laboratory activity is based on the qualitative mixing of cations and anions to form pigments. The materials given to each pair of students were worksheets, reagents to make their own colouring substances. Students filled the worksheets (see below) using drops of reagents. Two drops from each solution of each row reacted with two drops from each solution of each column. The demonstrator shows the principle of the experiment and lets 5 min to the student to complete the full table.

Slide 18: Complete the pigments table

Solution of iron(III) chloride, FeCl3, and potassium hexacyanoferrate(II) K4[Fe(CN)6] produces the pigment, Prussian blue. Mixture of Cobalt(II)chloride (CoCl2·6H2O) and sodium hydrogenphosphate (NaH2PO4·10H2O) produces the pigment cobalt violet. Mixture of AgNO3 and NaCl produces a white precipitate of AgCl, but it was never used as pigment because there are much cheaper silver-free white pigments (TiO2, chalk, lead white, zinc white). Students were told to add drops of reagents and observe the products. About 50 mL of each solution in dropper bottles was enough for all students.

  • Prussian blue: Iron(III)-hexacyanoferrate(II). The first modern, artificially manufactured color was Prussian blue. It was made by the color maker Diesbach of Berlin in about 1704. Diesbach accidentally formed the blue pigment when experimenting with the oxidation of iron. The pigment was available to artists by 1724 and was extremely popular throughout the three centuries since its discovery.
  • Cobalt violet: First prepared by Salvètat in 1859. It is chemically stable, but its rather high cost and low tinting power prevented its wider use in paintings. its drawbacks included high cost and weak coloring power, both of which limited its use among painters. It was quickly replaced by the cleaner, stronger pigment Manganese Violet.

Slide 19: Focus on Prussian blue

Mark Chagall is a famous Russian-French artist. He was a long user of Prussian blue in his paintaings. For him, blue was the colour of love (see painting on the left). The paintings of March Chagall are in great demand and are very costly. In 1992, a business man called martin Lang bought what he thought was a painting of Chagall. There was a lot of controversy to know it was a genuine Chagall’s paint. For this reason, experts analyzed the composition of the blue pigment. Pigment was identified as phtalocyanine blue and not the expected Prussian blue. However, phtalocyanine blue is a modern pigment and it wasn’t available as a commercial pigment at the beginning of 1900s, the period this watercolors is said to be dated. The outcome is that our business bought a fake! Thank you chemistry”.

ACTIVITY N°4: PREPARE YOUR OWN PAINT

The students work in trio. They receive a set of pigments (including azurite and malachite they have synthesized), binders and thinner. The teacher shows the preparation procedure then the students work successively on the four kinds of paints. A mortar and pestle is available to grind the pigment if necessary, there is a mortar for each pigment so don’t mix up. When all the samples have been mixed and tested, the results of the different groups can be examined by the whole class and compared. Each group can report on the mixing process, the application, the drying time and the finish. The following questions can be asked during the plenary:

  • How did you mix your paints? Did you have any problems?
  • Did you change the ratio of the mixture to make a good paint?
  • How did you apply the paint to the paper? Was the effect as you expected?
  • How quickly did the paint dry?
  • What is the finished effect of the paint like? Is it shiny, matt (not shiny), smooth, streaky, thick or thin?

At the end of the activity the groups can work on a painting using their colors.

ACTIVITY 4.1: OIL PAINT

  • Place a small spatula (made up wood) of pigment into a disposable cup.
  • Add a small amount of linseed oil.
  • Grind the oil and pigment with the stirring spatula.
  • Continue to mix, adding more oil if necessary to crush the aggregates.
  • As the pigments become more mixed with the oil, it becomes darker.
  • The mixed paint has a buttery consistency.
  • The oil paint is applied to “oil painting paper” paper with a cotton swab.
  • Thin the medium with turpentine as needed.

 

ACTIVITY 4.2: TEMPERA PAINT

  • Roll the separated egg yolk on a filter paper to absorb excess white.
  • Transfer the egg yolk to a disposable cup
  • Break the yolk sac with a spatula.
  • Remove the yolk sac with a spatula.
  • Mix the egg yolk (without the sac) with 1 mL of water (measured with a Pasteur pipette) and stir with your spatula.
  • Add a small spatula of pigment into a second disposable cup
  • Add water to the dry pigment to create a creamy paste.
  • Add an equal amount of egg yolk to the pigment paste. Thin the tempered medium with more water as needed.
  • Apply the paint to a regular paper with a cotton swab.

 

ACTIVITY 4.3: ACRYLIC PAINT

  • Place a small spatula of pigment into a disposable cup.
  • Add a small amount of water.
  • Once the pigments are completely dispersed, add the binder – in this case a matte acrylic gel medium.
  • Mix vigorously with spatula.
  • The result is an acrylic paint that is similar to manufactured acrylic paint.
  • Using a cotton swab., the acrylic paint is applied to a regular paper.

 

ACTIVITY 4.4: WATERCOLORS

  • A pile of pure ground pigment is place in a disposable cup with the aid of spatula.
  • Add gum Arabic solution.
  • Mix with spatula.
  • The result resembles watercolour from tubes.
  • The resulting watercolour paint applied to a regular paper using a cotton swab.

Tip: if the dispersion of the pigment is poor (grains or aggregates are clearly visible), it is advised to grind the pigment in a mortar.

 

Slides 20-21-22: Prepare your own paint

The demonstrator shows each time how to prepare the paint in front of the class starting from oil paint. The students can choose the pigment that they prefer. A discussion with the whole class concludes each activity.

 

Slide 23-24-25: Focus on linseed oil

For the most advanced classes in chemistry, the process of linseed oil cross-linking can be presented to the class.