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Alkalis are high pH and highly corrosive substances used for cleaning, paint stripping, mortar, drain clearing and other diy tasks. They have sizeable risks and require handling with due care.

Caustic soda

  • Strong alkali
  • Sodium Hydroxide
  • Cleans ovens
    • Foaming products help it cling
  • Unblocks drains
  • Strips alkyd gloss paint
  • Sometimes used to make soap, but potassium hydroxide is preferred
  • Toxic, irritant, can cause serious eye injury.
  • A relatively high risk chemical, follow instructions with care.
  • Causes the majority of household chemical burns
  • Its important to add crystals to water gradually, never the other way round, dissolution is strongly exothermic.
  • Use goggles, rubber gloves, plastic apron.

Paint stripping

Caustic soda strips oil based (alkyd) paints.

  • Products containing thickener are less likely to run off the workpiece, and more effective at cleaning or stripping on non-horizontal surfaces.
  • Paint stripping paste may be made with caustic soda thickened with any of:
    • builder's lime
    • wallpaper paste (excess caustic destroys the paste)
    • boiled flour (excess caustic destroys the paste)
    • toilet paper (laid on rather than mixed in)

Covering the paste with clingfilm keeps it working much longer

Sodium silicate

  • aka waterglass
  • dishwasher cleaner
  • once used to preserve eggs

Washing soda

  • Sodium Carbonate
  • Degreases when used with hot water.
  • Used for clothes degreasing & removal of some stains
  • Unblocks drains. The majority of drain blockages are mostly solidified fat. Half a cup of soda in hot water works well, but use eye protection as it can spit alkali when mixing
  • For very greasy laundry, a teaspoonful in the machine is plenty, with a hot wash.


Lime is made by burning stone to produce mainly alkaline calcium compounds.

  • Makes an array of paints
  • Used in lime mortar
  • Used as plasticiser in cement mortar
  • Used for lime plaster
  • Used to crisp pickles, and formerly used for preserving eggs
  • mainly calcium oxide & calcium hydroxide

Lime exists in several forms:

  • builder's lime: hydrated non-hydraulic lime, good for mortar etc
  • hydraulic lime: sets faster & harder, for more specialist uses
  • Quicklime: non-hydrated, vicious on skin, can sometimes burst into flame when soaked with water
  • Lime putty: made by mixing water with quicklime or builder's lime

Lime is sometimes sold as a desiccant to dry damp cupboards etc. Its not really effective, it does work in drying an airtight sealed container, but not a cupboard or room.

  • Quicklime is used to dispose of diseased animal carcasses, and does serious damage on contact with any part of the body. Its highly reactive and spits when slaked, sometimes catching fire.
  • Builder's lime usually doesn't burn at all, but occasionally causes severe burns right down to the bone. Be wary of skin contact, and avoid breathing in any powder.


  • Caustic
  • Mildly alkaline
  • Choking fumes
  • Avoid contact with eyes, skin and nose/lungs
  • Fumes in excess are seriously toxic
  • NH3
  • Dilute ammonia reduces the effects of acidic insect stings
  • Especially good at cleaning off congealed kitchen grease
  • Dissolves the fats & oils present in quite a lot of dirt & stain types
  • Destroys proteins
  • Glass cleaner, leaves a relatively streak free finish
  • Oven & hob cleaning, attacks the dreaded cooked fats
  • Stain remover for laundry etc
  • Antimicrobial, used to disinfect meat commercially
  • Refrigerant R717, but concerns of potential threat to life from possible release of the quantities used in minibar fridges have made it unpopular in domestic fridges
  • Ammonia fumes darken oak
  • Highly toxic to aquatic life, don't dispose of down a drain, ditch, lake etc
  • Avoid contact with bleach, it produces toxic chloroamines
  • aka spirit of hartshorn
  • Used in natural dying to alter the colours
  • Can be used to make a range of other chemicals
  • When used indoors, ensure plenty of ventilation or use a wet cloth breathing filter.
  • Fumes damage the imaging drums used in laser printers & many photocopiers


  • Sodium hypochlorite
  • pH 12.6
  • Consumes most types of household dirt, except limescale
  • Kills most bacteria, moulds and some other infectious agents
  • Very effective at cleaning silicone sealant. Apply folded loo roll & bleach, let soak overnight, and repeat a few times to get it looking like new
  • Thick bleach has added soap
  • Quickly removes tea stains on ceramics

Sodium bicarbonate

aka baking soda or bicarb

  • Technically this is a base rather than an alkali, and is mild & safe with many uses
  • Wetted bicarb works like toothpaste on teeth. Its abrasive to plaque but not to teeth, and is a stain remover, but lacks fluoride
  • Removes tea and coffee stains
  • Reduces laundry odour: add to final rinse
  • Removes black scuff marks from floors
  • Cleans plastic / fibreglass baths
  • Freshens sour dishcloths: soak in water and bicarb
  • Deodorises laundry awaiting washing: sprinkle in the basket.
  • Removes crayon marks: use a brush and soda paste.
  • Sometimes touted as a multipurpose cleaner, but better products exist
  • Used in cakes & soda bread


Alkalis can cause serious eye injury. The damage takes time to occur and isn't felt, so may not prompt a person to seek medical assistance. Major damage can occur, upto blindness. Always use effective eye protection.

Adding most alkalis to water generates heat, and boiling mixes can spit. Don't do it unless you're well protected against splashes.

Don't mix alkalis with acids, rapid boiling or violent reactions may occur, spitting hot acid or alkali. Some mixes can go well above 100C and melt through whatever its in, or shatter glass. The same can happen when adding hot water to strong alkali.

Strong alkalis should be kept off skin. Caustic soda and quicklime are particularly nasty, but others have been known to cause severe injury at times. If any skin contact occurs, the alkali should be washed off with copious amounts of water.

Due to these risks, the use of strong alkalis for paint stripping is objected to by some diyers, while its practised by some. The risks and long soaking time require prevention of any nonessential access to both humans and animals.

Alkalis in general can discolour or completely dissolve aluminium.

If contact with alkali occurs, the important thing is to act promptly. Failing to act right away increases the likely damage hugely. Sometimes a soapy feeling is noticed at first as the alkali converts body fat into soap.

See also