In this experiment, students learn how to make a lake pigment paint and analyse how the properties of the materials used have changed as a result of a chemical reaction
In this practical students will:
- Learn about how lake pigment paints are made, using key terms such as pigment, binder or liquid medium, lake pigment, organic, inorganic and fugitive.
- Prepare lake pigment paints, analysing any property changes and indications that a chemical reaction has occurred.
- Evaluate the quality of their paints on different surfaces, determining which binder produced the best paint.
Introduction for teachers:
This topic could start with a group discussion on everyday coloured substances and the components of paint. During this discussion the teacher introduces the following ideas especially the words in bold.
A paint is easy to make since a basic paint has only two components; the coloured part known as a pigment, mixed with a liquid medium (binder) that sticks the pigment to a surface.
The pigment can be a mineral or rock that keeps its colour, but sometimes a dye is used and this is known as a lake pigment. A lake pigment is a pigment manufactured by precipitating the dye with an inert (unreactive) ‘mordant’, which is usually a metal compound. This means that lake pigments are organic, unlike pigments from minerals which are inorganic.
A basic lake pigment is easy to make. The basic principle is to precipitate the dye, e.g. plant juice, on to aluminium hydrate or aluminium sulfate. For younger students this process can be described as the alum ‘grabbing’ the dye colour.
Artists refer to many lake pigments as being fugitive. This means they lose their colour in the light.
(This practical is best done as a science club activity since it takes a long time to complete and requires some experimentation to get the right conditions. It is best for students to work in groups of three to discuss the approaches.)
This practical is designed for secondary students and the aim is to gain some understanding of the way artists used scienctific methods to create pigments. It links with:
- setting up simple practical enquiries, comparative and fair tests;
- reporting on findings from enquiries and observations, including oral and written explanations, displays or presentations of results and conclusions;
- using straightforward scientific evidence to answer questions or to support their findings;
- building a more systematic understanding of the chemistry of paint by exploring the way coloured materials can be used to make a paint;
- asking questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience;
- using appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety;
- making and recording observations using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements;
- presenting observations using appropriate methods;
- interpreting observations and identifying patterns using those observations to draw conclusions;
- presenting reasoned explanations, including explaining data in relation to predictions and hypotheses; and
- learning about the concept of precipitation, chromophores and mordents.
Scenario for learners:
You are an ancient Greek science-artist who has been commissioned to paint a fresco of your patron’s daughter. Like most women of the time, she likes to wear brightly coloured clothes of blue, red and orange.
You want to be as accurate as possible with your painting and you wonder if it is possible to create paints using the same dyes used on the clothes. Like all good science-artists, you decide to investigate further …
Equipment (per group):
- 30 g potassium aluminium sulfate, also known as alum
- 25 g potassium carbonate IRRITANT
- 100 cm3 ripe buckthorn berries or juice or alternative(s) provided
- 2 beakers (250 cm3)
- 200 cm3 water
- 2 Bunsen burners
- 2 heat resistant mats
- 2 tripods
- 2 gauzes
- 2 thermometers (0-110 °C)
- 2 stirring rods
- 1 filter paper
- 1 filter funnel
- 1 conical flask and bung
- 1 jar with lid for the product
- 1 dropping pipette
- 1 mortar and pestle
- Kettle, hot plate, water bath
- Beaker or disposable plastic cup
- Egg yolk
- Linseed oil
- Spatula (or spoon)
- Take 100 cm3 of the buckthorn juice (the dye) (or the alternative provided for you) and filter it into a flask to remove any solids that are in the solution.
- Pour 200 cm3 of water into a 250 cm3 beaker. Place the beaker on a tripod and gauze, over a Bunsen burner on a heat resistant mat. Light the Bunsen burner and warm the water to 60 °C.
- Add 10 g of alum to the beaker of warm water and stir to dissolve as much as possible.
- Keep the solution at 60 °C and add the 100 cm3 of dye.
- Now pour 25 cm3 of water into another beaker. Place the beaker on a tripod and gauze, over a Bunsen burner on a heat resistant mat. Light the Bunsen burner and warm the water to 60 °C.
- Add 10 g of potassium carbonate to the warm water and stir to dissolve it. Be careful the potassium carbonate is an irritant so wash it off your skin if spilt.
- Very carefully and slowly pour the potassium carbonate solution into the dye solution. Make sure to do this gently since there can be some effervescence. It may be advisable to place the beaker onto a tray before doing this.
There should be some precipitation of the lake pigment.
- What gas do you think is being given off? How could you find out?
- Why do you think this chemical reaction occurred?
Leave the mixture to cool and settle. This is best if the contents are left overnight. (If time is short you can allow the solution to cool just enough to place into a cold water / ice bath.)
- When you look at the mixture after it has settled you will see the precipitate at the bottom.
- Where do you think the precipitate has come from
- Why do you think the precipitate has gone to the bottom of the beaker?
- Remove the excess liquid using a dropping pipette and then pour the remains through a filter funnel with filter paper and into a conical flask. This is a very slow process since the particles are small and can clog the filter paper.
- Once the mixture has been filtered, lay the filter paper out on a flat surface so that the precipitate can dry. The filter paper can be placed onto a white tile and this can be placed onto a hot plate. Needs supervision. It can then be scraped off and collected in a jar. This is the lake pigment.
- Make paint with the lake pigment by using a mortar and pestle to grind the dried powder, then mixing the fine powder with either egg yolk or linseed oil.
- Try painting on a piece of paper, a canvas and a piece of wood.
- Which is the best surface and if you used both egg yolk and linseed oil which made the best paint and why?
Students can evaluate the lake pigment for colour fastness with regard to light. This would require them to design an investigation.
They could also try using onion skins, flower petals or red cabbage to make a lake pigment.
The hazards in this practical are from boiling liquids and thorof ns on the plant. With younger students it is probably better to use the juice from the berries to save them extracting the juice and risk damage from the thorns.
The chemicals are not toxic but the potassium carbonate can be an irritant when made into a solution so plastic gloves are recommended. Also the juice can stain the skin.
Safety glasses should be worn during the practical.
If using dyes then avoid raising the dust or provide students with strong solutions of the dyes.
Whatever material is used, the extracted colour colours the alum and this can be used as a lake pigment mixed with a binder such as egg yolk or linseed oil. The potassium aluminium sulfate (or alum, AlK(SO4)2.12H2O) is the reagent that forms the solid (precipitates) as hydrated alumina bound to the dye. This is known as the lake pigment.
The metallic salt, in this case alum, is used as the binder or mordant for the lake pigment must be inert, colourless, chemically neutral and insoluble in the liquid medium. In the Greek and Roman periods alum, chalk, white clay, and crushed bones were often used as sources of the binder for lake pigments. In today’s industry the binders used for lake pigments are barium sulfate, calcium sulfate, aluminium hydroxide and aluminium oxide (alumina).
Many lake pigments are azo dyes. Lake pigment compounds generally have areas on the molecule that have a negative charge, often on the chromophore (the part of the molecule that gives it colour). The chromophore absorbs some wavelengths of light and reflects others that give the substance colour.
If using buckthorn you will get sap-green colour, if using woad it gives you blue, madder you get red and cochineal you will get crimson red. Changing the pH of the solution can change the hue and even in some cases the colour.
The ancient Greeks and Romans used madder to get a red and in 1815 Humphrey Davy described a range of Roman pigments and hinted at one being madder. This sample, now at the British Museum, has since been analysed and shown to be madder on a calcium sulfate (gypsum) substrate. Madder was also used by the Egyptians to dye cloth as far back as 1567 BCE. The lake pigments were important pigments for the Greek, Roman and Renaissance artists.
Alum is the most accessible mordant for students. The use of alum salt solutions is safe.
Any plant berry can be tried. However, historically the most common plants used for lake pigments are madder root, woad and cochineal food dye, which can all be bought on the internet. This experiment can also be carried out with more accessible materials, such as onion skins, flower petals or red cabbage to make the lake pigment.
The approach to making a lake pigment is similar for each material, but some dyes may need to be boiled while for others, such as madder, boiling can ruin the colour. This can only be found out by experimentation. Also the quantities of alum and alkali will vary depending on the dye being used so some experimentation will be needed to get the quantities right for the dye being used.
If you want to start with a plant one of the most common plants that can be used is the purging or common buckthorn (Rhamnus cathartica). Buckthorn can be found in woodlands in the UK and the purple-black berries can be collected at the end of the summer (Figure 1).
After collecting the buckthron berries, they can be crushed using two wooden boards - one on top and another below - over a bowl to catch the juice. Add some water into a bag and crush carefully. Cut a corner off so only the fluid runs into the bowl. The collected juice can be heated to concentrate it, but be very careful not to boil it. With younger students it is probably better to provide them with the concentrated juice to eliminate the risk of them hurting themselves on the thorns.
Whatever material is used the extracted dye colours the alum. This can be used as a lake pigment and mixed with a binder such as egg yolk or linseed oil. If using buckthorn you will get sap green, if using woad it gives you blue, madder you get red and cochineal you will get crimson red. These were important colours for the Renaissance artists.
If egg yolk is used as the binder, the pigments apply well, have a slight difference in colour due to the egg yolk and a slight sheen.
- Stays on paper;
- Scrapes off wood;
- Stays on canvas.
With linseed oil they have a good colour and a higher sheen.
- Doesn’t stay on paper;
- Wipes off wood;
- Stays on canvas.
Interested in seeing the final product? Go to this blog to see some of the steps of the practical and examples of the buckthorn berry lake pigment being used.