Mordants for Natural Dyeing

A mordant is required if the colour from natural dyes is to remain wash fast and permanently bonded to the fibre. Materials can be pre-soaked in a mordant and left damp prior to dyeing. The mordants can also be added to the dye bath while the dye is being simmered. Mordanting materials before and after dyeing with natural dyes will add extra fastness. Visit our Dyeing section to browse for all the Natural Dyeing and Mordant products mentioned in this post.

Dyeing without Chemicals?

The quantities of mordant used in preparing fibre for dyeing have been the subject of much disagreement over recent years. Historically the craft dyer used ‘enough’ to make sure the job was done well. This gave rise in recent years to a strong reaction for ecologically sound mordanting where the levels of mordant were reduced considerably, alternative mordants were sought out, and additives used to increase the effectiveness of mordants. The most extreme version was the concept of ‘dyeing without chemicals’. The belief that oxalic acid used as a mordant and derived directly from rhubarb leaves and roots is ‘good’ and the same substance from a factory is ‘bad’, is prejudiced. Oxalic Acid is a listed poison!

The most commonly used mordants are Alum (Potassium Aluminium Sulphate), Copper (Copper Sulphate), Tin (Stannous Chloride), and Iron (Ferrous Sulphate). These are used in various combinations with assistants for mordanting wool, silk and other protein fibres. The process of mordanting cotton, linen and other cellulose fibre for natural dyeing is different in that the mordants do not stick to the fibre. The cellulose fibre needs first to be soaked in a solution of tannin so that a mordant can attach itself to the tannin.

Before dyeing, it is important to check whether the item is ‘Prepared for Dyeing’ (PFD) or requires scouring to remove any grease, oil or starch. Run a few droplets of cold water onto the fabric. If they soak in quickly, no scour is necessary. To remove starches, size and oils, add 5ml of Synthrapol (a non-ionic detergent) along with 2-3 litres of water for each 100g of material. Stir gently over a 15 min period, and then rinse thoroughly in warm water. It is possible to use household detergent, but the alkaline residue may affect the final colour or wash fastness.

Mordant Pack for Natural Dyeing
A good way to experiment with mordants is with the Fibrecrafts Mordant Pack which includes Alum, Copper, Tin, Iron and Cream of Tartar

Mordants for Dyeing Wool and Silk (Protein) Fibres

The quantities of mordants shown are for 500g of dry weight of fibres with five litres of water. Bring the mixture to boiling temperature and hold there for one hour. Do not agitate as this will felt the fibres. Allow to cool, rinse and dry.

  • Alum Mordant: 100g, it helps to add 25g of Cream of Tartar* (Potassium Hydrogen Tartrate). Use of more than 150g of Alum will tend to make the fibre feel sticky.
  • Copper Mordant: 100g, add 20g of Cream of Tartar to improve colour and light fastness.
  • Tin Mordant (Stannous Chloride): Use a small amount with natural dyes to brighten colours, especially yellows, oranges and reds. Use 3-5g Tin to 500g dry fibre weight – too much Tin will make the fibre brittle.
  • Iron Mordant: 20g in solution is added to the dye bath after dyeing to darken the dye colour.
  • Titanium Oxalate: 50g, brings out oranges from tannins such as Gall Nut and variable colours from other natural dyes.

Experimenting and careful note taking will help you to repeat the colours and results achieved by these mordants. *Cream of Tartar is used to soften fibre, to act as a buffer to maintain a slightly acidic dye bath, to help disperse the Natural Dye colour evenly and to brighten colours (i.e. taking cochineal to a true red) on wool, silk and other protein fibres. It is an optional addition to the Alum mordant solution at 3-5g per litre of water.

Cotton and Linen (Cellulose) Fibres

Dyeing cotton and linen with natural dyes requires a different approach to that used for protein fibres. The metallic part of the mordant does not attach readily to cellulose until it has reacted with Tannic Acid (tannin).

  • First dissolve 40g of Tannic Acid in 5 litres of hot water. Steep the fibres in the bath for up to 24 hours.
  • Mordant the prepared fibres with a mix of 250g of Alum and 25g of Soda Ash (sodium carbonate) in 5 litres of hot water and allow to soak for up to 24 hours, rinse in warm water and allow to dry before repeating the mordanting. Copper and Tin mordants can be used in similar proportions as for wool to assist with blues, blacks and reds.

Alternatively, Aluminium Lactate acts as an effective mordant for Cotton (and other cellulose fibres) when using natural dyes, giving a significant difference in colour take-up and improved light fastness. On cotton use 7-10% to Weight of Fabric (WOF) after a bath of gallnut (or other tannins of light colour) for a better fixation of the mordant. Also use in combination with iron mordant or a finishing bath for shading with an iron salt (max 1.5% WOF) to widely increase the number of shades. Titanium Oxalate can also be used in the same way as Aluminium Lactate for mordanting cellulose and will effectively bring out the orange in the tannin.

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