Experiments with natural dyes
The range of dye colours that can be obtained from nature is remarkably wide. However, many natural fibres like cotton and wool won't accept a dye without being treated with chemicals called mordants (from the Latin mordere meaning to bite).
Mordants and modifiers
Mordants are usually more effective if applied to the fibres before dyeing, but post-mordanting (applying the mordant after dyeing) can also produce good results. For the same fabric and dye, different mordants will often produce different shades, and in some cases startlingly different colours.
The colour of dyed fabrics can also be altered (sometimes quite significantly) by chemicals called modifiers. Dye colours can also be sensitive to the mineral content and acidity of the water used, the growing conditions of the plant used as the dye source, when it was harvested, whether it's used fresh or dried, and on the dyeing process used.
Doing it yourself
In this activity, we’re going to use onion skins to dye cotton a range of colours, from beige through copper to green-brown. We'll first use unmordanted fabric and then go on to pre-mordant the cotton with (i) alum and (ii) a mixture of alum and cream of tartar, to see if this makes any difference.
We'll also test what effect the use of three different modifiers (citric acid, washing soda and ferrous sulfate) has on the process.
Deeper colours can often be obtained by leaving the fabric to soak overnight in the dye-bath, or by dyeing it twice.
This activity involves a number of different steps, and you won’t necessarily be able to complete it in one session. However, many of the steps can be carried out simultaneously so as to save time. You may find it useful to split the activity into two sessions. In the first, you could clean and pre-mordant the fabric, and prepare the dye-bath. In the second session, you can then carry out the dyeing, and test what effect the modifiers have.
- To protect them from stains, spills and splashes, cover all of the work surfaces you’ll be using with newspaper.
- You must wear rubber gloves and an apron throughout this experiment.
- It is essential that you read the safety instructions associated with any chemicals before you use them. These are usually printed on the container.
What you'll need
- at least 36 strips of (preferably bleached) cotton(4cm x 10cm) – old, white, cotton pillowcases are ideal
- 25g coloured, outer onion skins (from about 3kg of small onions)
- washing soda (sodium carbonate)
- washing-up liquid
- ferrous sulfate (try and buy this chemical in 100mg tablet form – it’s used in the treatment of anaemia)*
- citric acid*
- cream of tartar*
- a number of heat-resistant containers of various capacities
- some form of tongs
- rubber gloves and apron
* available from most pharmacies
Record all of your observations and experimental procedures in a notebook as you go along. Throughout this activity, you’ll find that it’s very easy to lose track of what you’re doing, and to get confused about which strip of fabric is which. You’ll therefore need to keep a careful note of what’s going on.
Do not under any circumstances let your solutions boil dry. For much of the time you’ll be watching pots boil. Be sure to keep an eye on them, and replace the water that’s lost through evaporation so that the level of liquid is maintained.
All quantities and times given below are approximate.
Cleaning the fabric – simmer all of the cotton strips in two or more litres of water to which a teaspoonful each of washing soda and washing-up liquid have been added. While this is taking place you should prepare the dye-bath (see below). After two hours, remove the strips using the tongs, and rinse them thoroughly in cold water before simmering for a further hour in a fresh batch of water containing washing soda and washing-up liquid. Again, rinse them thoroughly and leave to soak in cold water until you come to use them.
Preparing the dye-bath – stirring occasionally, simmer the onion skins in two litres of water for two hours.
Mordanting the fabric – dissolve 2g of alum, and about a third as much of washing soda, in 400cm³ of boiling water (1cm³ is the same as 1ml). The gas given off is carbon dioxide. Using tongs, place 12 of the cleaned, wet cotton strips in 200cm³ of this hot alum solution, simmer gently, with occasional stirring, for one hour, and leave to cool and soak overnight.
To see what effect using a different mordant has, repeat the above process with another 12 cleaned, wet cotton strips. This time, however, dissolve a pinch of cream of tartar in the alum/washing soda solution before adding the fabric.
After mordanting overnight - remove the cotton strips, rinse them thoroughly in cold water, squeeze out any excess liquid and dye them straightaway. Alternatively, you could dry them in a warm place and store them dry for future use.
Dyeing the fabric – simmer 12 strips of cleaned, unmordanted fabric in the dye-bath (still containing the onion skins) for two hours (although it’s better, after simmering, to leave the strips to soak overnight in the cooled dye-bath).
Repeat the process using the strips of fabric pre-mordanted with alum and alum/cream of tartar. In each case, compare the colours that you get with those obtained using unmordanted fabric.
Modifying with citric acid – take one of the unmordanted, dyed strips and rinse it thoroughly in cold water. Simmer it for one hour in 100cm3 of boiling water to which a pinch of citric acid has been added. Stir the solution occasionally. Remove the fabric, rinse with cold water and leave it to dry.
Repeat the process using strips of fabric pre-mordanted with alum and alum/cream of tartar. In each case, compare the colours that you get with those obtained using unmordanted fabric.
Modifying with washing soda – repeat the above modifying process, but this time add a pinch of washing soda to the water instead of citric acid.
Modifying with ferrous sulfate – the effect of ferrous sulfate as a modifier is quite dramatic, and even a cold, very dilute solution will produce a result. Add about 10mg of ferrous sulfate to 100cm3 of cold water and use this as the modifier on the unmordanted and mordanted fabric.
Leave the strips to soak in this solution, stirring occasionally, for 1 hour, then rinse with water and dry. You'll need to estimate very roughly how much 10mg corresponds to: if your ferrous sulfate is in the form of 100mg tablets, 10mg corresponds to 1/10th of a tablet. Don’t worry, the amount you use need only be very approximate.
The range of colours that can be obtained from onion skins. Those in the top row are unmordanted, those in the middle row were obtained using an alum mordant, and those in the bottom row an alum/cream of tartar mordant. From left to right, the fabrics were mordanted with washing soda, citric acid and ferrous sulfate. The strips in the far-right column were unmordanted.
Repeat the above procedure, but this time use about 100mg of ferrous sulfate and simmer for 5 minutes. You will see what effect different modifying conditions have.
Repeat the procedure once more. This time use about 500mg of the ferrous sulfate and simmer for 1 hour.
You could try the following variations to see if you can produce a wider range of shades or colours:
- Mordant the fabric twice before dyeing. After mordanting once, do not rinse the cotton strips, simply squeeze them out and allow them to dry. Then wet the strips with water, before adding them to a freshly prepared mordant solution and repeating the mordanting process. Be sure to rinse the strips well before dyeing.
- Mordant the fabric before and after dyeing.
- Use onions with red skins. You could also experiment with other natural dye sources. Table 2 makes some suggestions.
- Try different mordants and/or modifiers.
- Experiment with different dyeing, mordanting and modifying temperatures, times and quantities.
Use different fabrics.
Be sure to record all of your observations as you make them, so that you can compare the results obtained using these different strategies.
Book 4 of ST240, Our Chemical Environment, The Open University
Dean J., Wild Colour, Mitchell Beazley
Edmonds J., The History and Practice of 18th-Century Dyeing
Fibrecrafts catalogue, available from 1 Old Portsmouth Road, Peasmarsh, Guildford, Surrey
Ingamells W., Colour for Textiles: A user’s handbook, Society of Dyers and Colourists
Barrow J. D., The Artful Universe, Oxford University Press
A quite remarkable book that will change the way you view the world. Extremely accessible.
Burton et al., Chemical Storylines, G. Heinemann Educational Publishers
Part of the Salters Advanced Chemistry course, which explores the frontiers of research and the applications of contemporary chemistry. For A level and other science courses aimed at 16 to 19-year olds.
Fraser A. and Gilchrist I., Starting Science (Book 1), Oxford University Press
Part of an integrated science course for the National Curriculum Key Stage 3 and Scottish Environmental Studies (science) for S1 and S2.
Northedge A. et al., The Sciences Good Study Guide, The Open University
Indispensable for students of science, technology, mathematics and engineering. Packed with practical exercises and activities, all aimed at making studying more enjoyable and rewarding. Lots of hints and tips for those returning to study.
Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace
An excellent and informative reference source for all kinds of real-life applications of chemistry. Explores the world of chemistry that surrounds us in our daily lives, explained in terms that everyone can understand. ‘Makes chemistry come alive.’
PS547 Chemistry for Science Teachers course materials, The Open University, 1992
A course designed for use by science teachers from a wide variety of backgrounds, with varying experience of teaching science. A familiarity with some basic science (perhaps physics or biology) is assumed, but little understanding of chemistry is required. The mathematical understanding needed for the course is not demanding.