फिटकरी (Alum (potassium aluminium sulphate))

Alum is potassium aluminium sulphate, a double salt that forms clear, glassy crystals and is sold as lumps or white powder. In India it is known as fitkari or phitkari (फिटकरी). It tastes sour and slightly astringent, and it dissolves much better in hot water than in cold.

• Formula: KAl(SO4)2·12H2O (the common crystal form)
• CAS number: 7784-24-9
• Molar mass: about 474.4 g/mol
• Melting point: about 92 °C, where it melts in its own crystal water

In water it releases aluminium ions that pull fine clay and dirt particles together into heavier flakes. That is why a bucket of muddy water clears after a few hours with a little fitkari.

Azlok sells alum powder in three pack sizes: 200 g, 400 g and 600 g. All three are general-purpose (technical) material, meant for household, craft and workshop use. The 600 g pack is described by the supplier as non-ferric, meaning it is low in iron, which matters if you use it as a dye mordant because iron dulls colours.

None of these packs is sold as food grade or as a medicine. Check today's price per kg to compare packs.

• Household: settling mud and silt in cloudy stored water before it is filtered or boiled.
• Crafts and textiles: mordant for natural dyes such as madder, turmeric and marigold on wool and silk, so the colour holds better.
• Personal care: traditionally rubbed on skin after shaving, and pressed into deodorant stones.
• Education: growing large clear crystals in school and home science experiments.
• Industry: leather tawing, paper sizing and as a coagulant in water treatment.

Settling cloudy water. Swirl a small lump, or a pinch of powder dissolved in a cup of water, through a bucket of muddy water for about a minute. Leave it still for three to four hours. Pour off the clear top water without disturbing the sludge. This only clears the water. It does not make it safe to drink, so still boil or filter it.

Mordanting wool or silk. A widely used home-dyer recipe:

1. Weigh the dry fibre. Use alum at 10 to 15% of that weight, so 10 to 15 g for 100 g of wool.
2. Dissolve the alum in a little hot water, then add it to a pot with enough water for the fibre to move freely.
3. Add wetted fibre, heat slowly to about 80 to 85 °C and hold for about an hour. Don't boil wool, it felts.
4. Let it cool in the pot, rinse lightly and go to the dye bath.

Cotton needs a different method, usually a tannin step first, so this recipe gives weak colour on cotton.

Crystal growing. Stir alum into very hot water until no more dissolves, pour the clear liquid into a clean jar, hang a seed crystal on a thread and leave it undisturbed for several days.

• Alum dust irritates the eyes and nose. Avoid shaking powder near your face, and rinse eyes with clean water if it gets in.
• Don't eat it. Large amounts swallowed cause vomiting and stomach upset.
• Use gloves for long dyeing sessions, as the solution dries the skin.
• Keep it away from strong alkalis such as caustic soda.
• Store in a closed jar in a dry place. It is fairly stable but cakes in humid weather.
• Keep away from children, who may mistake the clear lumps for mishri.
• Small amounts of used mordant water can go down the drain with plenty of water, or onto acid-loving plants in small doses.

For occasional water settling or a single crystal project, the 200 g pack lasts a long time, since you use very little each time. If you dye regularly, pick the 600 g non-ferric pack: at 10 to 15% of fibre weight, a kilo of wool alone needs 100 to 150 g of alum. The 400 g pack sits in between for steady household use.

If you need alum for anything eaten or for medical use, these packs are not the right product. For workshop, dyehouse or water-treatment quantities of 25 kg and above, send a bulk inquiry with the quantity and how often you need it.