We can add potash alum to the water to purify it. Sand, clay and other suspended impurities of water will settle down. Water will be pure after decantation.
Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound: the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystallizes in cubic structure with space group P a -3 and lattice parameter of 12.18 Å[4]. The compound is the most important member of the generic class of compounds called alums, and is often called simply alum.[5]
Potassium alum is commonly used in water purification, leather tanning, dyeing,[6] fireproof textiles, and baking powder as E number E522. It also has cosmetic uses as a deodorant, as an aftershave treatment and as a styptic for minor bleeding from shaving
In antiquity
Potassium alum was known to the Ancient Egyptians, who obtained it from evaporites in the Western desert and reportedly used it as early as 1500 BCE to reduce the visible cloudiness (turbidity) in the water.
Potassium alum was described under the name alumen or salsugoterrae by Pliny,[8] and it is clearly the same as the stupteria described by Dioscorides.[9] However, the name alum and other names applied to this substance — like misy, sory, chalcanthum, and atramentum sutorium — were often applied to other products with vaguely similar properties or uses, such as iron sulfate or "green vitriol".[10]
The production of potassium alum from alunite is archaeologically attested on the island Lesbos.[11] This site was abandoned in the 7th century but dates back at least to the 2nd century CE.
Potassium alum is mentioned in the Ayurveda with the name phitkari or saurashtri.[citation needed] It is used in traditional Chinese medicine with the name mingfan
Potassium alum was imported into England mainly from the Middle East, and, from the late 15th century onwards, the Papal States for hundreds of years. Its use there was as a dye-fixer (mordant) for wool (which was one of England's primary industries, the value of which increased significantly if dyed).
These sources were unreliable, however, and there was a push to develop a source in England especially as imports from the Papal States were ceased following the excommunication of Henry VIII.[12]
Historically, potassium alum was used extensively in the wool industry[13] from Classical antiquity, during the Middle Ages, and well into 19th century as a mordant or dye fixative in the process of turning wool into dyed bolts of cloth.[citation needed]
In the 13th and 14th centuries, alum (from alunite) was a major import from Phocaea (Gulf of Smyrna in Byzantium) by Genoans and Venetians (and was a cause of war between Genoa and Venice) and later by Florence. After the fall of Constantinople, alunite (the source of alum) was discovered at Tolfa in the Papal States (1461). The textile dyeing industry in Bruge, and many other locations in Italy, and later in England, required alum to stabilize the dyes onto the fabric (make the dyes "fast") and also to brighten the colors.[14][15]
With state financing, attempts were made throughout the 16th century, but without success until early on in the 17th century. An industry was founded in Yorkshire to process the shale, which contained the key ingredient, aluminium sulfate, and made an important contribution to the Industrial Revolution. One of the oldest historic sites for the production of alum from shale and human urine are the Peak alum works in Ravenscar, North Yorkshire. By the 18th century, the landscape of northeast Yorkshire had been devastated by this process, which involved constructing 100 feet (30 m) stacks of burning shale and fuelling them with firewood continuously for months. The rest of the production process consisted of quarrying, extraction, steeping of shale ash with seaweed in urine, boiling, evaporating, crystallisation, milling and loading into sacks for export. Quarrying ate into the cliffs of the area, the forests were felled for charcoal and the land polluted by sulfuric acid and ash