Research by Robin Paris
Published in the Batik Guild Magazine, March 2008
Soda Ash. Na2CO3. Sodium Carbonate. Washing Soda
Call it what you will, soda ash is a traditional chemical of our dyeing craft. As an alkali it produces optimal dyeing conditions by raising the pH (ie alkalinity) of fibre-reactive dye enabling cellulose fibre (eg cotton) to form a permanent bond with the dye. Cellulosics will not dye at low pH (ie acidic conditions), though silk can be dyed at high pH with fibre-reactive dyes or low pH with acid dyes.
But soda ash is no newfangled chemical – it has history. Before soda ash, potash (potassium carbonate) was the European alkali of choice, extracted from wood fire ash with water. From the second millennium BCE ashes of certain salt-tolerant plants were used to produce alkali, including saltworts, glassworts, mangroves and seaweeds. Plants were harvested, dried and burned. An alkali solution formed by washing the ashes with water was then filtered and boiled dry to produce impure soda ash. Sodium carbonate concentration varied greatly from around 3% in kelp ash to 30% in the best quality Spanish saltwort ash.
Soda ash, like potash, was used predominately in the making of glass and soap. It was in use in dyeing in Europe by AD 50. Spain was Europe’s main source until the nineteenth century – its soda ash was known as barilla (from Spanish for ‘saltwort’). The saltwort Salsola soda was a major cultivated species, hence the name ‘soda ash’. In the eighteenth century Scotland, France, Ireland and Norway had major industries producing soda ash from seaweed (known as kelp) – at its peak Orkney was exporting 3000 tonnes of kelp ash annually. Glasswort was also used in France. But plant-derived soda ash couldn’t keep up with eighteenth-century European demand, and chemists began researching artificial production.
Differences between potash and soda ash were recognised but not fully comprehended until 1807, when Cornish chemist Humphry Davy isolated a metal from molten wood ash (potash) which he called potassium, and another metal from molten soda ash which he named sodium. Sodium and potassium do not exist in a free state in nature but only in combination with another element – they are extremely reactive elements that react vigorously with water. Their relatively simple atomic structures give them strong bonding reactions but also they create molecules which can separate into ionic mode in water. Sodium ions can interact with other molecules and affect their chemical behaviour. With Procion MX dyes, sodium ions interfere in hydrolysis of the dye (dye bonding to water) and with chemical bonds on the cellulose fibres, encouraging the two to link up – to dye the cellulose.
Lye is a corrosive alkaline substance, commonly sodium hydroxide (NaOH, also known as ‘caustic soda’) or historically potassium hydroxide (KOH, from hydrated potash). Previously, lye was among the many different alkalis leached from hardwood ashes. Today lye is commercially manufactured using a membrane cell method.