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Woodworking : How to make your own strong casein wood glue from milk
Wikimedia picture - Stockholm Central Station uses Casin Glues.
Warning: Always add acid to water and not the other way around otherwise you may get a violent reaction (boiling splashing nasty action).
Always handle chemicals according to the manufactures' recommendations, and any other precautions that you can find. Use gloves and eye goggles. Avoid burns and spills, and reactions with incompatible objects. Don't mix chemicals unless you are well aware of the result before doing it. Some chemical mixes can burst into flame or explode or boil. Do not consume chemicals in case they are poisonous. Prepare your workspace appropriately, have an assistant. Use protective clothing, and know how to contact the poisons centre for advice in an emergency.
In short - proceed at your own risk.
What ? Glue Just From Milk?
Almost. You need to separate the casein. For that, a mild acid, and some heat will do the trick.
Also, you will need to use skim milk powder.
Casein - What is it?
Casein is a protein. It is sometimes called caseinogen and you will find it in about 80% of the proteins in milk. Technically, it is a salt of the element calcium which, incidentally is a metal. So you should see why it is likely to be pretty tough stuff. You might even find it as an ingredient in nail polish.
It is not coagulated by heat. So how do you extract it from milk? The answer is to use acid. You may be interested to know that a proteolytic enzyme called rennet which is found in the stomachs of calves will also extract casein. ( Think about that next time you enjoy non-vegan prepared cheese - but remember that it is reportedly good for your teeth.)
Casein is fairly hydrophobic which means that it is not water soluble and that's a nice property for wood glue.
About the glue.
In ancient China, craftsmen used skimmed milk and rennet with a little lime to produce a very good water resistant glue. You can do something similar using just skimmed milk and vinegar. We will explore this later.
Although it is quite water resistant, total permanent immersion will break down the bond. However, so will microorganisms. Hence you might find that a commercial preparation also contains a fungicide but for non damp internal and even load-bearing structures, you can skip the fungicide.
Is it used much today?
No. Formaldehyde based glues are just as strong but more moisture resistant. This is a shame because casein glues have proven to last 50 or 60 years. For example, the Stockholm Central Station in Sweden was built between 1867 and 1871 using casein glue for huge laminated arches. Also WW2 fighter aircraft had casein glue structures. Commercial preparations started probably in the 1800s in Germany or Switzerland. They were used in the USA until about 1916 (ish).
Should it be used more today?
Yes. Casein glues could be manufactured for much less than the cost of modern equivalents. Formaldehyde is a nasty chemical but simple casein glue without lime or fungicide is not poisonous. Concentrated lime can burn skin - so only use that with extreme caution.
- Skim milk powder.
- White vinegar.
- Baking soda.
1. Dilute skim milk powder as per packet instructions to make 100 mL. Put into a pan. Add 15% vinegar (15 mL for 100 mL of liquid). [ For information: vinegar is 5% acetic acid ]
2. GENTLY heat and stir. As soon as it curdles, remove it from the heat. If you heat too much it will ruin the glue.
3. Pore the liquid and curds through a paper filter. You can use a funnel and a paper towel
4. Wash the casein to remove traces of vinegar.
5. Add 1/2 a teaspoon of baking soda to the curds and carefully stir with a small flat stick. Add water one drop at a time while stirring, until you reach a typical glue-like texture.
If you want to create a casein powder for use with various formulae, then after point 4. above press the casein in a paper towel to remove most of the water and let it dry hard. Then you can reduce it to powder. Do not mix the alkali before this stage because over time the powder will lose its properties.
The quality of the glue depends on low fat and acid content. This is why you start with skimmed milk and why you wash the vinegar away. Bi-carbonate of soda is an alkali and of course will neutralize the vinegar.
Substitute lime for bi-carbonate of soda.
Substitute sodium hydroxide for bi-carbinate of soda for a long life high strength glue but sacrifice some water resistent properties because sodium hydroxide is hydroscopic which means it will 'suck in' water molecules.
If you would like to make a prepared powder which also contains the alkali, then you can use calcium hydroxide. This is not hydroscopic but when you eventually add water it will turn to sodium hydroxide.
- sodium oxalate,
- sodium tartrate,
- sodium citrate,
- sodium salicylate,
- sodium phosphate,
- sodium sulfite,
- sodium fluoride,
- sodium arsenate,
- sodium arsenite, or
- sodium stannate
The following is known to work, but take appropriate care in every case:
- copper salts,
- mercury salts,
- the chlorinated phenols or their sodium salts to 5% by volume.
By weight (any units) - assume powder.
Then add either:
Sodium hydroxide 11
CaIcium hydroxide 20
Some notes on the ingredients:
Accurate weigh scales will give a better result than by volume.
Sodium hydroxide (NaOH) is "caustic soda", also known as Iye and sodium hydrate. It must be handled with care since it will burn skin.
Using the glue.
A runny glue has a longer work-time and it is easy to spread but too much water negatively affects strength. Glue will stain some woods and too much water causes more staining.
A jelly-like consistency will be strong. It has a short work-time and may be difficult to spread.
You should aim for a warm-honey consistency.
Clamp the wood for 2 to 4 hours. If the temperature is high, then it will dry faster. Hard wood gluing needs more time than soft wood. You can machine it after 6 hours or so.
The glue is quite hard and will tend to blunt your tools, so be sure to make them sharp and keep them sharp.
See this article about sanding wood.