Paint Pigments

Introduction

What is Paint?

Paints have a difficult job to do. They must be liquid in order to be applied but they must dry to a hard, impervious finish. The pigments must cover the underlying material completely and have a color which does not fade in strong sunlight or due to exposure to air or water. If a paint solidifies in a can it’s no good. Paints consist of a pigment, a solvent or thinner, and a binder. The pigment provides color, the solvent makes it a liquid to begin with, and the binder allows it to dry to a hard film. When the paint is applied the solvent evaporates and the binder cures, holding the pigment in place.⁽¹⁾ Two main types of paint are in common household use. The first, and up to about 1950 the only, kind of paint is oil-based paint. The most common binder in oil-based paint is so-called ‘boiled’ linseed oil. The oil is never boiled (oils do not boil) but it is heated and purified prior to its use in paint. Without this pretreatment, linseed oil will behave like any other oil: it will spread out and make an oily stain. The second common type of paint is latex paint, which is water-based. The latex is an acrylic resin which hardens when the water and other ingredients evaporate.

Artist’s paints may be oil-based, acrylic (a form of latex paint), water-color, or tempera. There are of course other painting techniques but these four are most familiar. Water color paints may be made using water, pigment and gum arabic. Tempera paints may come to mind in connection with grade-school art classes. Big bottles of brightly colored liquid that are used to make paintings that your parents hang on the fridge. These paints use an inorganic glue as a binder and are more properly called poster paints. Egg tempera is a very old painting medium and was the main medium used by artists up until the development of boiled linseed oil-based paints in the early 1500s. To make egg tempera paint the yolk of an egg, without its membrane, is mixed with water and a pigment.⁽²⁾ Generally, the paint is made fresh and used immediately though some commercial preparations exist that have a limited shelf-life. Traditionally, tempera paint is applied to a wooden board treated with gesso. Gesso is a mixture of white pigment, water, and glue. The white pigment is usually chalk (CaCO₃) or gypsum (CaSO₄).⁽³⁾ In this classroom activity you will paint on a piece of hard water color paper.

Pigments are the heart and soul of painting. Oil, acrylic, or egg yolk determine the painting techniques that are appropriate but it is the pigments that determine what colors can be made. Pigments are insoluble colored materials. They do not dissolve in the dispersing medium, unlike dyes.⁽⁴⁾ There are several kinds of chemical substances that may serve as pigments. They may be organic and of animal, vegetable, or synthetic origin. They may also be inorganic, synthetic substances or natural minerals.⁽⁴⁾ For a long-lasting color there is nothing better than an inorganic, colored salt of a transition metal. Some metals give rise to a wide variety of colors useful to an artist. These may be different compounds, different crystalline forms of the same compound, differently sized crystals of the same form, or the same compound with different degrees of hydration. The element chromium (element number 24) gets its name from the Greek χρωμα, chrōma, which means color. Various compounds of this element are used as pigments for yellow (citron or zinc yellow, ZnCrO₄ or mineral lake, Sn(CrO₄)₂, and chrome yellow, PbCrO₄), green (viridian or chrome green, Cr₂O₃), and red and orange (chrome red and chrome orange, PbCrO₄).⁽⁵⁾ In this lab activity we will not be making chromium-based pigments because of their toxicity but they are, many of them, in common use in artistic painting. Iron is the element at the heart of Prussian Blue (more about which later) and also many, many earth tones. Based on the oxidation number of the iron ion (+2 or +3) and the degree of hydration earth tones vary from yellow to red, green, brown, ochre, sienna, umber and terre verte.⁽⁴⁾ Some earth pigments will also be discussed later.

How Pigments Make Colors

The production of colors is a fairly complex topic. Colors can arise from the absorbance and transmittance of different parts of the color spectrum, from the interference of light waves with one another upon reflection, and from the scattering of light by small particles. With inorganic pigments, colors arise due to the selective absorbance of some colors of the spectrum and the reflection or transmittance of others. White materials, such as titanium dioxide, reflect a large percentage of all colors of light that falls on it. Titanium dioxide is white and very opaque because all colors are reflected equally and strongly. When all parts of the visible spectrum are reflected at equal intensity we see white. Black materials, such as lamp black or charcoal, absorb a large percentage of all colors of light. Since light falling on black material is neither reflected nor transmitted the material is black. A colored pigment such as Prussian blue selectively absorbs yellow, orange and red wavelengths of light and reflects only blue. In this lab activity you will make several pigments: malachite, Prussian blue, two different earth tones, and lamp black. In addition, as a white pigment to adjust the shade of your paints, you will use titanium dioxide.

Copper Carbonate Hydroxide

This pigment makes a sea green color known as malachite. Malachite is a naturally occurring mineral that has been used to make paints since ancient times with evidence of its use in Egypt’s fourth dynasty in 3000 B.C.E.⁽⁶⁾ The chemical composition of malachite is copper carbonate hydroxide (CuCO₃·Cu(OH)₂) and it is a copper ore found in several locations worldwide. It is created in the earth by reactions with water and carbon dioxide in copper-containing rocks exposed to the elements.⁽⁷⁾ A closely related mineral, azurite, has a blue color.

In this lab activity copper(II) carbonate hydroxide will be synthesized by reacting copper(II) sulfate (CuSO₄) with sodium carbonate (Na₂CO₃). The reaction makes a mixed precipitate of two copper compounds due to the fact that the carbonate ion is a weak base, which produces hydroxide ions (OH^–) in solution. The overall reaction is as follows:

Equation 1          2CuSO4 + 2Na2CO3 + H2O → CuCO3·Cu(OH)2 + 2Na2SO4 + CO2

This reaction benefits from being kept cold because the crystals that form then remain very small and therefore suitable for dispersion in a medium to make paint. If heated too strongly the CuCO₃·Cu(OH)₂ will decompose to produce water vapor and carbon dioxide and CuO. This pigment cannot be dried in an oven.⁽⁹⁾ The color of this pigment in oil paints is darker and reportedly it works best in egg tempera paint. The tiny particles of the precipitate tend to cling together and a significant amount of grinding in a mortar will be required in order to make a paint that does not have a grainy texture.

Prussian Blue

This versatile material has many uses and one of them is as a strong blue pigment. A half-ounce of the pigment is enough to give a blue color to give 20 pounds of lead white (PbCO₃·Pb(OH)₂)⁽⁵⁾ a blue color.⁽⁸⁾ Prussian blue is considered one of the first modern synthetic pigments. It was first discovered by Heinrich Diesbach in Berlin around 1704. Kept a secret for twenty years, the traditional synthesis of Prussian blue is complex, difficult, and smelly. Published in England by Woodward in 1724 the process involves heating equal parts potassium nitrate (KNO₃) and potassium tartrate (K₂C₄H₄O₆) in a red-hot crucible. Dry, powdered cattle blood is added and heated until it glows. The resulting material is washed with alum and iron(II) sulfate (FeSO₄), which produces a green solid. This turns blue when hydrochloric acid (HCl) is added.⁽⁸⁾ Needless to say, this would be inconvenient to carry out in a student laboratory.

A metal coordination compound is one that forms between a metal ion and atoms or molecules which donate pairs of electrons to create a bond between themselves and the metal ion. Prussian blue is likely the first metal coordination compound ever synthesized and is the origin of the word cyanide for the CN^– ion. The carbon and nitrogen in the proteins of the blood used in the original synthesis were broken down by the high heat to make the cyanide ion, though this was not known in this way at the time. When the ion was isolated and its structure understood the association between it and this blue pigment is why it was called cyanide: κυανοσ, kyanós, means dark blue in Greek.⁽¹²⁾

The process used in this activity is much simpler and uses purified chemical starting materials. Iron(III) nitrate (Fe(NO₃)₃) or iron(III) chloride (FeCl₃) may be used as a source of iron(III) ions (Fe³⁺) in solution. A saturated solution is preferable. A saturated solution of potassium hexacyanoferrate(II) (also called potassium ferrocyanide, K₄Fe(CN)₆) is prepared and then mixed with the solution containing Fe³⁺ ions. Instantly upon mixing, a dark blue, very fine precipitate forms. Sources vary with respect to its exact chemical formula but it is likely Fe₄[Fe(CN)₆]₃ (see equation 2).⁽¹¹⁾

Equation 2        4FeCl3 + 3K4[Fe(CN)6] → Fe4[Fe(CN)6]3 + 12KCl

The product can be filtered and washed and placed in a warm oven to dry (not above 100°C). If dried in this way it is likely that the product will be a hydrate with 14 to 16 molar equivalents of water.⁽¹⁴⁾ This synthesis procedure produces material fine enough that little grinding should be necessary before mixing with water and egg yolk to make tempera paint. It is a very dark blue and a small amount of pigment will go a long way. It can be mixed with titanium dioxide, which will be provided as an additional pigment for this activity. The titanium dioxide (TiO₂) will provide good covering power and will reduce the translucency of the paint. It may also lead to temporary bleaching upon exposure to strong light.⁽⁹⁾

Prussian blue is the same pigment that makes traditional blueprints blue. Copy machines being scarce before 1949, it was ordinarily necessary to make multiple copies by hand or by making an expensive metal plate for use in a printing press. The materials to produce Prussian blue by exposure to ultraviolet light being inexpensive they were used to make copies of the plans for big projects. A transparent sheet was prepared with the detailed technical drawings. This was laid over a large piece of paper treated with the chemical sensitizer and exposed to sunlight or another source of UV. This process can also be used to make sun prints of plants, flowers, or other objects or even to make photographic prints using suitable transparent negatives. See one example at left above.

Iron Oxide Earth Pigments

For millennia this group of mineral pigments has been used for purposes as diverse as cave paintings, cosmetics, and painting barns. Many rocks and soils contain iron oxides as iron is one of the most common metals in the Earth’s crust. There are four iron oxide minerals used in pigments: hematite, limonite, siderite and magnetite. Hematite is anhyrdrous iron(III) oxide (Fe₂O₃) and is an earthy red color. Other iron ores, when heated to a high temperature, convert into hematite. Limonite is a hydrated iron oxide and gives rise to the artist’s pigments ochre, sienna, and umber. The chemical formula of limonite is Fe₂O₃·n H₂O or FeO(OH)·n H₂O. The number of water molecules that are incorporated into the crystalline structure can vary, which affects the observed color of the material. The color ranges from bright to dark yellow.⁽⁴⁾ In this lab activity you will first make a hydrated iron pigment, which has a dark orange-brown color. Then, by heating it strongly in a covered crucible you will calcine the material and remove the water of hydration, producing a red hematite pigment. The initial reaction is between iron(III) ions and hydroxide ions which produces an insoluble hydroxide, as in equation 3.

Equation 3        FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl

The chemistry of this reaction appears to be straightforward but I have not yet come across information that says exactly what is going on. It is only clear, so far, that no such material exists which has the formula Fe(OH)₃.⁽¹³⁾ Instead, it must be some form of hydrated iron oxide. You will make it part of the scientific work of this lab to determine the empirical formula of the precipitate you make.

Lamp Black

Pure carbon exists in several different, allotropic forms. The most expensive allotrope of carbon is, of course, diamond. Diamonds, when pure, are perfectly clear crystals. If made into a powder suitable for making a paint pigment it would be white. But it would be exceedingly costly. For far less money it is possible to make pure carbon in a kitchen or chemistry lab. Simply burn a carbon-based fuel such as kerosene or candle wax using a wick and place a piece of glass into the bright part of the flame. As a part of the process of combustion, the fuel loses its hydrogen atoms before completely combining with oxygen to form carbon dioxide. This produces an amorphous form of carbon with a wide variety of molecular structures. Most of these are fused six-membered rings that look like scraps of chicken wire.

Carbon collected in this way is known as lamp black because it is often made using a lamp with a wick. The material is extremely fine and the small particle size makes it excellent for making fine inks such as India ink. It never fades and has excellent coverage. Since it is a hydrophobic material it may not mix well with water. The use of a co-solvent, such as ethanol, can make it easier to form a paste for incorporation into a paint or ink.

Titanium Dioxide

This pigment is the only one not being manufactured in this lab activity. It is a naturally occurring mineral which is the primary source of the metal titanium. Suitably purified titanium dioxide (TiO₂) is used in cosmetics, sunscreen (it is an excellent UV absorber), foods, and paints. It is also useful in the lab and industry as a photocatalyst and applications are being developed for its use in solar panels to generate electricity. Titanium dioxide can be made into coatings which are self-cleaning: they absorb sunlight and destroy dirt and bacteria that adhere to the surface.

As a pigment the hiding power of titanium dioxide is unrivaled. To better understand why, it is helpful to consider Kirchoff’s Law: the intensity of incident light on a material equals the combined intensity of the light that is reflected, transmitted, and absorbed by it. (I_incident = I_reflected + I_transmitted + I_absorbed). The higher the refractive index of a material is, compared to its surroundings, the higher the percentage of light reflected from it. Titanium dioxide has a very high refractive index and as a result of this a large fraction of the light that strikes it is reflected.⁽¹⁰⁾ Such a large fraction of light is reflected by titanium dioxide that there is very little left for transmission. Unlike white pigments such as whitewash (CaCO₃), titanium dioxide allows very little light to be transmitted from the surface that it coats.⁽¹⁰⁾

Pre-lab Questions

Answer these questions before coming in to the lab. Submit your answers using a Google Doc. Use the “Insert Equation” function to show your work for calculations. Do not do your work on paper and insert a photo of it. You must each do your own independent work on these questions.

1. Read these two sources about the art of tempera painting.
   http://www.webexhibits.org/pigments/intro/tempera.html
   http://www.artyfactory.com/art_appreciation/art_movements/italian-renaissance/italian-renaissance-art-tempera-painting.html
   Write a brief summary of what you have learned. (No more than two paragraphs but be sure to include some juicy details).
2. Look at Equation 1 in the introduction, which shows the synthesis of malachite pigment. Why is carbon dioxide produced in this reaction? Here’s how to work it out:
   1. Write a net ionic, balanced chemical equation for the reaction between copper(II) ions and the carbonate ion.
   2. Write a net ionic, balanced chemical equation for the reaction between the carbonate ion and water to produce two moles of hydroxide ion in the balanced equation.
   3. Write a net ionic, balanced chemical equation for the reaction between copper(II) ions and hydroxide ions.
   4. Write a balanced chemical equation for the decomposition of carbonic acid.
   5. Add these equations together, eliminating formulas which cancel out from one side to the other.
   6. Summarize in words what happens in solution when sodium carbonate solution is added to copper(II) sulfate solution.
3. In the procedure for the synthesis of malachite it calls for 6.25 g of copper(II) sulfate pentahydrate (CuSO₄·5H₂O, molar mass 249.68 g/mol) and 3.7 g of sodium carbonate monohydrate (Na₂CO₃·H₂O, molar mass 124.00 g/mol). Using the balanced chemical reaction below, determine which of these is the limiting reactant.
   2CuSO₄ + 2Na₂CO₃ + H₂O → CuCO₃·Cu(OH)₂ + 2Na₂SO₄ + CO₂
   Also, what is the predicted yield of the pigment in grams?
   
   
4. In the procedure for the synthesis of Prussian blue pigment it calls for 9.2 g of iron(III) chloride hexahydrate (FeCl₃·6H₂O, molar mass 270.29 g/mol) and 3.0 g of potassium ferrocyanide trihydrate (K₄Fe(CN)₆·3H₂O, molar mass 422.39 g/mol). Using the balanced chemical reaction below, determine which of these is the limiting reactant.
   4FeCl₃ + 3K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ + 12KCl
   Also, what is the predicted yield of the pigment in grams?
   
   
5. In the procedure for the synthesis of iron oxide pigment it calls for 12 g of iron(III) chloride hexahydrate (FeCl₃·6H₂O, molar mass 270.29 g/mol) and 5.5 g of sodium hydroxide (NaOH, molar mass 40.00 g/mol). Using the balanced chemical reaction below, determine which of these is the limiting reactant.
   FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl
   Also, what is the predicted yield of the pigment in grams?
   
   
6. Each precipitation reaction involves colored solutions, which are mixed together. Write a net ionic equation for each one. Since you have identified the limiting reactant you will be able to predict which ions are completely used up and which will have some left over. Based on these predictions, for each of the precipitation reactions in turn describe how you will be able to tell whether or not the reaction has completely used up the limiting reactant in each case. Consider the colors of solutions of iron and copper ions as you compose your answer.
7. When you wash the iron oxide earth pigment precipitate what ions will you be washing off? In other words, what are the spectator and excess ions in the reaction? Your net ionic equation from the previous question will be helpful here. Based on your answer, how should the pH of successive washes change as you rinse it with fresh water at least three times?
8. Prussian blue is a coordination compound which contains an ion in which an iron(II) ion is bound to six molecules of the cyanide ion. Coordination chemistry is a topic that does not come up in our usual curriculum in high school chemistry, though we do cover the related topic of Lewis acids and bases. Read through these two sources about coordination compounds to learn more about this type of compound.
   http://www.chem.purdue.edu/gchelp/cchem/ and
   https://chem.libretexts.org/Core/Inorganic_Chemistry/Coordination_Chemistry/Properties_of_Coordination_Compounds/Coordination_Compounds
   Now, write a brief description of this type of compound. It should be no longer than a paragraph and should be sure to define several key terms.

Materials

Safety

• If you choose not to wear safety glasses you are choosing to sit out the lab.
• Copper(II) sulfate pentahydrate (CuSO₄·5H₂O) is toxic and its dust is an eye and respiratory tract irritant. Avoid contact and wash hands after handling it. If exposed to dust, remove victim to fresh air. If in eyes, rinse with cold water for up to 15 minutes.
• Iron(III) chloride (FeCl₃·6H₂O) is toxic and its dust is an eye and respiratory tract irritant. Avoid contact and wash hands after handling it. If exposed to dust, remove victim to fresh air. If in eyes, rinse with cold water for up to 15 minutes.
• Potassium ferrocyanide trihydrate (K₄Fe(CN)₆·3H₂O) is slightly toxic by ingestion. Avoid contact and wash hands after handling it.
• The dust from solid sodium carbonate (Na₂CO₃) is an eye and respiratory tract irritant. It is also toxic by ingestion. Avoid contact and wash hands after handling it. If exposed to dust, remove victim to fresh air. If in eyes, rinse with cold water for up to 15 minutes.
• Sodium carbonate is a strong base and can cause severe damage to skin and eyes. Avoid contact and wash hands after handling it. If using gloves, adhere to proper glove procedure. If in eyes, rinse with cold water as soon as possible for up to 15 minutes.
• The dust from solid sodium hydroxide (NaOH) is an eye and respiratory tract irritant. It is also toxic by ingestion. Avoid contact and wash hands after handling it. If exposed to dust, remove victim to fresh air. If in eyes, rinse with cold water for up to 15 minutes.
• Sodium hydroxide is a strong base and can cause severe damage to skin and eyes. Avoid contact and wash hands after handling it. If using gloves, adhere to proper glove procedure. If in eyes, rinse with cold water as soon as possible for up to 15 minutes.
• When mixing sodium hydroxide with water a large amount of heat will be released. This can cause the water to boil and the liquid to spatter. Sodium hydroxide can cause blindness if it enters the eyes. Adhere to instructions carefully to mitigate this hazard.
• Denatured alcohol is toxic by ingestiong and is an eye irritant. If in eyes, rinse with cold water as soon as possible for up to 15 minutes.
• Denatured alcohol is highly flammable. Keep flames and sources of ignition far from acohol.
• If anything goes wrong with the bunsen burner immediately close the lab bench fuel valve
• If you cannot get the burner to light within about 15 sec then immediately close the lab bench fuel valve and wait for assistance
• Never ever leave the burner unattended!
• Keep flammable substances far away from the burner
• Be careful not to touch objects recently heated by the burner: hot glass looks just like cold glass!
• Flames and hot objects can cause fires and burns: use caution at all times
• Always close containers of chemical supplies when they are not in use.
• Wash hands thoroughly with soap and water before leaving the laboratory.
• Do not consume the egg yolks as they are high in cholesterol.

Making Pigments

Be aware of the chemical hazards as you follow this procedure. If done carefully there is little chance of harm. If carried out carelessly permanent injury could result. Have fun!

Each lab group will make just one kind of paint pigment. The amounts used are enough for groups to share pigments with other groups once the products of your lab work are ready to be put to use to make art.

Malachite

This is a pigment made from the co-precipitation of copper(II) hydroxide (Cu(OH)₂) and copper(II) carbonate (CuCO₃).

1. Measure about 6.25 g of copper(II) sulfate pentahydrate (CuSO₄·5H₂O) on a lab balance. Transfer to a 250-mL Erlenmeyer flask. (If the mass is not exactly 6.25 g, that’s OK. Just measure it precisely.)
2. Put a magnetic stir bar in the flask. Dissolve the copper sulfate in 25 mL of water by running the stirrer at low speed.
3. Measure 25 mL of water into a 100-mL beaker. Using a lab balance, measure 3.7 g of sodium carbonate monohydrate (Na₂CO₃·H₂O) and add it to the water in the beaker. Stir to dissolve.
4. The reaction between copper sulfate and sodium carbonate produces carbon dioxide gas. Mixing the two together too quickly can result in the production of foam which may overflow the flask. Add the sodium carbonate solution slowly to the flask containing the copper sulfate solution while stirring constantly at a low speed.
5. When the reaction is complete (that is, when the bubbling ceases) stop the stirrer and remove the stir bar.
6. After the reaction is complete it can be filtered to collect the precipitate. Follow your instructor’s directions to fold a piece of filter paper into a fluted shape. This shape increases paper surface area in contact with the mixture to be filtered and allows air pressure to equalize above and below the wet paper.
7. Weigh the filter paper and record its mass. When you weigh your pigment later you will do so with it sitting on the paper and you will need to subtract the paper’s mass.
8. Set the filter paper in an appropriate funnel. Support the funnel above a 250-mL beaker. Be careful not to overfill the filter paper! Carefully pour the liquid part of the reaction mixture into the filter paper. Follow with the precipitate. Wash any solids in the flask into the filter using a wash bottle.
9. Once the filtration is complete, which may take up to 20 minutes, wash it with fresh water. Cover it with fresh water and allow it to drain through completely. Repeat the washing process so that the precipitate is washed at least three times. Then carefully pull the filter paper out of the funnel and lay it on a watch glass. Allow the precipitate to dry for up to a week before making final measurements. The malachite pigment may not be heated to dry it because when heated it decomposes to form CuO.
10. In order to calculate stoichiometric yield, weigh a container in which the pigment may be stored. Scrape the dry product off the filter paper into the container and weigh it. The transfer may be aided by the use of a powder funnel. Use the information to calculate the % yield for the chemical reaction.
11. When making this pigment into a paint it will be necessary to grind it with a mortar and pestle so that the paint does not have a grainy texture.

Prussian Blue

This pigment (Fe₄[Fe(CN)₆]₃) is a deep blue color. Although it contains cyanide ions (CN^–), Prussian blue is not poisonous.

1. Measure about 9.2 g of deep-orange iron(III) chloride hexahydrate (FeCl₃·6H₂O) and dissolve it in about 20 mL of water in a 50-mL beaker. Stir to dissolve. The amounts given here should make a saturated solution. (If the mass is not exactly 9.2 g, that’s OK. Just measure it precisely.)
2. A saturated solution of potassium ferrocyanide trihydrate (K₄Fe(CN)₆·3H₂O) may be prepared by measuring about 3.0 g of the yellow crystals and dissolving them in about 20 mL of water in a 50-mL beaker. (If the mass is not exactly 3.0 g, that’s OK. Just measure it precisely.)
3. Mix the two solutions together in a 100-mL beaker and stir. The dark blue precipitate will form immediately.
4. Follow your instructor’s directions to fold a piece of filter paper into a fluted shape. This shape increases paper surface area in contact with the mixture to be filtered and allows air pressure to equalize above and below the wet paper. Set the filter paper in an appropriate funnel. Support the funnel above a 250-mL beaker.
5. Weigh the filter paper and record its mass. When you weigh your pigment later you will do so with it sitting on the paper and you will need to subtract the paper’s mass.
6. Be careful not to overfill the filter paper! This precipitate is very fine and will settle only very slowly. Pour the mixture into the filter paper a little at a time. Wash any solids sticking to the reaction beaker into the filter using a wash bottle.
7. Once the filtration is complete, which may take up 20 or more minutes, wash it with fresh water. Cover it with fresh water and allow it to drain through completely. Repeat the washing process so that the precipitate is washed at least three times. Then carefully pull the filter paper out of the funnel and lay it on a watch glass. Allow the precipitate to dry for up to a week before making final measurements. Strong heating may release cyanide gas from this material so heating to dry is not recommended. Low temperatures may be useful to speed the drying process. Otherwise, it may take up to a week to fully dry.
8. In order to calculate stoichiometric yield, weigh a container in which the pigment may be stored. Scrape the dry product off the filter paper into the container and weigh it. The transfer may be aided by the use of a powder funnel. Use the information to calculate the % yield for the chemical reaction.
9. When using this pigment to make paint only very tiny amounts will be needed as it makes a very dark color. Mixing with the white pigment titanium dioxide will allow for a range of blue hues to be made.

Hydrated Iron Oxide

In this procedure you will proceed as if making the chemical iron(III) hydroxide (Fe(OH)₃). This species does not exist. Instead, there are in fact several variations on the empirical formula FeO(OH)·nH₂O. Depending on the crystal structure and degree of hydration this material can take on a variety of earth colors. You will be making two earth colors.

Hydroxide Precipitate

1. Measure about 12 g of deep-orange iron(III) chloride hexahydrate (FeCl₃·6H₂O) and dissolve it in about 25 mL of water in a 250-mL beaker. Stir to dissolve. (If the mass is not exactly 12 g, that’s OK. Just measure it precisely.)
2. Put on a pair of nitrile or latex gloves to protect your hands. Be sure that if sodium hydroxide gets onto the gloves you do not touch things with the contaminated gloves.
3. Measure 5.5 g of sodium hydroxide (NaOH) pellets using a lab balance. This should be in slight stoichiometric excess over the iron(III) chloride.
4. Adding water to sodium hydroxide releases a large amount of heat. This is a special hazard because it can come to a boil and spatter strongly alkaline solution. To avoid this you will mix the sodium hydroxide in the water in a very gradual fashion. First, measure 25 mL of water using a graduated cylinder and pour it into a 100-mL beaker.
5. Add a small amount of dry sodium hydroxide to the water in the beaker and stir using a glass stirring rod. Stir until it is completely dissolved before adding more NaOH. Note the rise in temperature. Monitor this temperature and allow the solution to cool a bit between additions. By adding only a little at a time and stirring each time until it dissolves it should be possible to keep the temperature relatively low.
6. Once the sodium hydroxide has fully dissolved, add the solution to the iron(III) chloride solution. Stir to mix. Once the reaction is complete and you have rinsed out the beaker that contained the NaOH you may remove the gloves.
7. Follow your instructor’s directions to fold a piece of filter paper into a fluted shape. This shape increases paper surface area in contact with the mixture to be filtered and allows air pressure to equalize above and below the wet paper. Set the filter paper in an appropriate funnel. Support the funnel above a 250-mL beaker.
8. Weigh the filter paper and record its mass. When you weigh your pigment later you will do so with it sitting on the paper and you will need to subtract the paper’s mass.
9. Be careful not to overfill the filter paper! This precipitate is almost gelatinous in character. It settles only very slowly, if at all. Carefully pour the reaction mixture into the filter paper. Wash any solids sticking to the reaction beaker into the filter using a wash bottle.
10. Once the filtration is complete, which may take more than an hour, wash it with distilled water. Cover it with water and allow it to drain through completely. Repeat the washing process so that the precipitate is washed at least three times. Then carefully pull the filter paper out of the funnel and lay it on a watch glass. Allow the precipitate to dry for up to a week before making final measurements. Heating in an oven for a few hours at about 80°C is an effective way to speed the drying process.
11. Once it is dry, scrape the product off the filter paper into a pre-weighed container and weigh it.
12. This material makes a fine brown-earth pigment but the color may be changed to a dark red or orange by heating it strongly in a crucible to drive off water of hydration. This is called calcining.

Calcining the Precipitate

1. Use about half of the product of the previous procedure to complete this one. Measure the amount by mass.
2. Write down the exact mass of the sample you will calcine. By doing so you should be able to use the mass, after driving off water, to determine the empirical formula of the original precipitate. Do this based on the assumption that all mass lost is due to water according to this equation: Fe₂O₃·nH₂O(s) → Fe₂O₃(s) + nH₂O(g).
3. Set up a bunsen burner, ring stand with ring, and clay triangle. Transfer your weighed sample of hydrated iron oxide to the crucible, place it on the clay triangle and heat to red hot. To increase the internal temperature, use a cover for the crucible. Be careful to handle the cover and crucible only with crucible tongs!
4. Only a few minutes will be necessary to effect a complete dehydration of the material. Heat for up to five minutes then turn off the bunsen burner and allow the material to cool.
5. Weigh the product so that you can use the change in mass to deduce the empirical formula of the hydrated iron oxide produced in the precipitation reaction. According to your assumption the final product is anhydrous iron(III) oxide (Fe₂O₃).
6. Store the cooled, calcined pigment in a labeled bottle or vial.

Lamp Black

Lamp black is a pigment with a long history. The incredibly fine particles of pure carbon make an excellent pigment in oil paint and in tempera.

1. Begin by gathering a candle, a lighter, a ring-stand, a clamp, a 500-mL Erlenmeyer flask, some ice and some water.
2. Weigh the candle to determine its mass. By doing so, and weighing it at the end of the collection process, you will be able to calculate the efficiency of the carbon black production process.
3. Set up these items as shown in the image at right. The flask is clamped to the stand above the candle and filled with ice water. When lit, the candle flame must just touch the bottom of the flask so that it produces a black deposit of soot. By carefully adjusting the height the production of soot can be maximized.
4. Allow the candle to burn in this way for 10 - 15 minutes. Collect the soot by scraping it into a suitable pre-weighed dish. Replace the flask over the candle and adjust again to maximize soot production. Replace the ice water if all of the ice has melted. Do not use an empty flask: the result of doing so is broken glass.
5. Repeat this process until about 0.02 - 0.05 g of soot have been collected. The time required will vary but may be in excess of 30 minutes.
6. Record the burn time of your candle and the mass yield of your ‘lamp’ black. Weigh the candle again to determine its final mass so that you can calculate the amount of candle wax that was consumed.

Titanium Dioxide

This pigment will be provided to you ready-made. It is a white pigment with very good coverage properties and can be used to alter the shade of the other colors made in this lab activity.

Making Paint

Tempera paint is made using egg yolk, pigment and water.

1. Only make as much paint as you think you are likely to use. This paint cannot be stored and used on another day. That being said, if you are making a blend of pigments it may be useful to make more of the pigment blend than you will use in one sitting. The dry mixture can be saved and used to make more paint later.
2. Start by grinding the pigment in a mortar and pestle, if needed. The malachite and iron oxide pigments in particular will require a rather long grinding time to be fine enough to disperse evenly. The lamp black and Prussian blue should not need any significant grinding, though the Prussian blue may have fused into lumps and may need to be broken up.
3. Put a small amount of pigment into a dish or onto a suitable surface. Add water to the finely ground the pigment until you have a uniform paste. The soot may not mix well with water. Add a few small drops of ethanol to help it to mix. Good dispersion is critical to making good paint.
4. Obtain an egg and a 250-mL beaker. Break the egg and let the white of the egg drain into the beaker; retain the yolk in the shell. Pour the yolk back and forth between the halves the shell to remove more of the white. Alternatively, this can be done by carefully holding the yolk over the fingers and allowing the white to drain between the fingers.
5. Gently place the yolk in its membrane on a paper towel. By lifting up the edge of the paper towel allow the yolk to roll to the edge. This helps to remove any remaining egg white. Egg white in the paint will cause it to drag on application and dry more quickly.
6. When the yolk is at the edge of the paper towel pierce it with a toothpick or the end of a paperclip and let the yolk drain out of the membrane into a 50-mL beaker.
7. Add about 5 mL of water to the yolk and stir. It is now ready to be mixed with pigment.
8. To make paint add together equal amounts of pigment paste and egg yolk mixture. Mix thoroughly. To obtain the consistency that you want you may add water.
9. A useful painting technique is to create layers of paint. The layers can be the same or different colors. Some paints will be more translucent or transparent than others; use this to your advantage. The paint will allow itself to be painted over once it is dry. Too much water can disturb the dried paint.

Painting

The whole reason we make pigments is so that we can use them to make art! Typically, tempera painting is done on wooden boards but a stiff sheet of paper will do for our purposes. Paint can be applied over pencil drawings and can be built up in layers to create effects of different degrees of transparency and a layered build-up of colors.

Perhaps some people are not feeling creative. Discuss with your teacher some alternative activities. For example, compare the hiding ability of the different pigments to each other of when using different amounts of water mixed with the egg yolk. Or, make a color test sheet showing the different shades that can be achieved by mixing the colored pigments with white or black or with one another: how wide is the range of colors you can achieve with the pigments you made?

Your teacher may make provisions to allow you to express yourself with paint over several class meetings. Plan accordingly.

Your teacher will provide a place to display your in-progress and completed artwork. You may take your work home when you are finished with it or you may donate it to your teacher.

Post-lab

Answer the following questions in a typed document. Each student must do their own work, independently. Show work for calculations using the “Insert Equation” function.

1. Using the limiting reactant for your chosen pigment synthesis, what is the theoretical (or expected) yield for your pigment? Report the number of grams of pigment that should be produced if all of the limiting reactant is used up.
2. What was the actual yield in grams for the synthesis of your chosen pigment? In other words, what was the final measurement of the mass of the pigment in grams? This can only be determined once the pigment has completely dried.
3. Calculate a percent yield, as (your collected mass)/(predicted yield) × 100%. Report your results and discuss reasons why the yield may not have been exactly 100%.
4. You calcined some portion of the dry iron hydroxide precipitate. Or someone in your class did and provided you with the relevant data. Based on the mass of the product of this heating process, and the fact that the final formula of the material is simply Fe₂O₃, what is the empirical formula of the dried iron hydroxide precipitate? Assume that all of the lost mass can be accounted for as molecules of H₂O.
5. How much wax was consumed in the production of lamp black from a candle? How many grams of black pigment were produced and collected? What is the efficiency of conversion calculated as (mass of pigment)/(mass of candle burned) × 100%? Comment on the factors that contributed to increasing or decreasing the yield of carbon black. These may have been things under your control such as positioning of the apparatus or things not under your control such as the diameter of the candle or the size of the wick.
6. Please submit a digital photo of your artwork with your report. You may keep your artwork to bring home or you may donate it to your teacher.
7. Artists are capable of producing things of beauty, though that is not the sole purpose of art. Are scientists capable of producing things of beauty, even though it is the purpose of science to create new knowledge? In other words, can an experiment, a theory, or a discovery be beautiful? What do you think?