Alum – what is it?

On my next walk along the Derwent River – Stage 12, I will start by walking 3-4 kilometres along the top of the Alum Cliffs from Hinsby Beach to Kingston. I have seen the cliffs from Hinsby Beach on Stage 11, and they look like ‘normal’ cliffs.

But what is Alum? 

According to http://chemistry.about.com/od/moleculescompounds/f/What-Is-Alum.htm, “Alum is any of the compounds with the empirical formula AB(SO4)2·12H2O” and if I tell you Alum is a specific compound of hydrated potassium aluminium sulfate with the formula KAl(SO 4)₂·12H 2O, your brow may wrinkle.  Are you still in the dark?  Worse – it seems there are a number of alum varieties including:

  • Potassium alum is also known as potash alum or tawas. It is aluminium potassium sulfate. This is the type of alum that you find in the grocery store for pickling and in baking powder. It is also used in leather tanning, as a flocculant in water purification, as an ingredient in aftershave and as a treatment to fireproof textiles.
  • Soda alum, with the formula NaAl(SO4)212H2O, is used in baking powder and as an acidulent in food.
  • Ammonium alum has the formula NH4Al(SO4)212H2O and is used for many of the same purposes as potassium alum and soda alum. Ammonium alum finds applications in tanning, dyeing textiles (acts as a mordant and helps to lock in colours), making textiles flame retardant, in the manufacture of porcelain cements and vegetable glues, in water purification and in some deodorants.
  • Chrome alum or chromium alum has the formula KCr(SO4)212H2O. This deep violet compound is used in tanning and can be added to other alum to grow lavender or purple crystals.
  • Selenate alums occur when selenium takes the place of sulphur, so that instead of a sulfate you get a selenate, (SeO42-). The selenium-containing alums are strong oxidizing agents, so they can be used as antiseptics, among other uses.

Other uses of Alum

Alum has several household and industrial uses.

  • purification of drinking water as a chemical flocculant
  • in styptic pencil to stop bleeding from minor cuts
  • adjuvant in vaccines (chemical that enhances immune response)
  • deodorant “rock”
  • pickling agent to help keep pickles crisp
  • flame retardant
  • the acidic component of some types of baking powder
  • an ingredient in some homemade and commercial modelling clay
  • an ingredient in some depilatory (hair removal) waxes
  • skin whitener
  • ingredient in some brands of toothpaste

Considering this list, and in this day and age of developers raping resources, I am surprised our Alum Cliffs are still standing.

The more I learn the more I am overwhelmed by options.  I wonder what the story is of our Alum Cliffs between Hinsby Beach and Kingston. More research is required.

4 thoughts on “Alum – what is it?

  1. chantale delrue

    Ahhh alum…. Just got a 25 kg bag of the stuff…for my dyeing… Alum Cliff walk is nice… Walked on the beach at Dodges Ferry today… What a Summer we are having! Just like August in Belgium…. Grey and wet… How are you going otherwise? Must catch up sometime soon…

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