Dyeing Blue-Purple with Red Cabbage — The Kitchen-Chemistry pH Indicator Dye

Use one medium red cabbage (approximately 500-800 g) for 100 g of wool. Chop the cabbage into thin strips or rough chunks — smaller pieces release pigment faster. The outer leaves are richest in anthocyanins — include them. The cabbage should be a deep purple-red throughout. Pale or mostly-green cabbages give weak results.

Place the chopped cabbage in a dye pot with 4 litres of water. Bring slowly to 70-75°C and hold for 30 minutes. Do not boil — excessive heat degrades the anthocyanins. The water will turn a vivid blue-purple as the pigment dissolves. Stir occasionally. Red cabbage releases its dye rapidly — the colour change is dramatic within the first 10 minutes.

Strain the dye liquor through a fine mesh strainer into a clean pot. Press the cabbage to extract all remaining purple liquid. The spent cabbage will be pale and limp. The strained liquor should be a vivid blue-purple — this is the natural pH of the cabbage extract (around pH 6-7). Adding vinegar turns it red. Adding baking soda turns it blue-green. This pH responsiveness is the hallmark of red cabbage anthocyanins.

Pre-wet the alum-mordanted wool in lukewarm water for 15 minutes, squeeze gently, and lower it into the red cabbage dye bath at room temperature. Slowly raise to 65-70°C over 15 minutes, then hold for 45 minutes. Turn gently every 10 minutes. The wool will develop a blue-purple to lavender tone. The alum mordant slightly shifts the pH and produces a bluer result than un-mordanted wool. Experiment with pH modifiers after dyeing: a brief dip in vinegar water shifts the colour toward pink, while a dip in soda water shifts toward blue-green.

Turn off the heat and let the wool cool in the dye bath overnight. Remove, squeeze gently, and rinse in lukewarm water until the runoff is mostly clear. The final colour is a soft blue-purple to lavender. Lightfastness is poor — red cabbage anthocyanins fade significantly within weeks of sunlight exposure. Washfastness is also limited. This makes red cabbage unsuitable for garments or items exposed to light, but ideal for educational dyeing, craft projects stored indoors, and understanding dye chemistry. The colour's pH sensitivity makes every skein a chemistry experiment. Dry in shade.