Make filter charcoal from your own fire

Every survival diagram of a homemade filter has a charcoal layer in it. Most of them are drawn by people who have never tested the output, and the layer does something quite different from what the diagram implies.

Charcoal is genuinely worth having. It is worth having for the right reasons.

What charcoal does and does not do

The taste point is not trivial, though. In a long emergency, water people will not drink is not water. Children in particular will refuse water that smells of pond, and a dehydrated child is a real problem. Charcoal solves that, and that alone earns its place.

Charcoal is not activated carbon

Worth being clear about, because most published claims for "charcoal filters" are actually claims about activated carbon.

Activated carbon is charcoal that has been treated at high temperature with steam or chemicals to blow open its internal pore structure, giving an enormous internal surface area, sometimes over 1,000 m² per gram. That vast surface is what does the adsorbing.

Home-made charcoal from a wood fire has some of this structure and far less of it. It works, at perhaps a fraction of the capacity, and it exhausts much sooner.

Treat home-made charcoal as a taste and smell polish with modest chemical adsorption, and treat bought activated carbon as the real thing. If you can stock activated carbon, do; it is cheap, stores indefinitely dry, and is far more capable.

Choosing the wood

• Hardwood beats softwood, clearly. Oak, beech, ash, hazel, birch, fruit woods. Denser wood gives denser charcoal with more structure and it lasts longer in use.
• Softwood works at a pinch but is lighter, dustier and exhausts faster. Pine also carries resin, which is why it is the second choice.
• Never use treated, painted, varnished or preserved timber. Tanalised or pressure-treated wood contains copper and, in older stock, arsenic and chromium. Burning it concentrates those into the charcoal and you would then be running your drinking water through them. This includes old fence panels, decking, pallets of unknown origin and most reclaimed timber.
• Never use plywood, MDF, chipboard or anything glued. The adhesives are not something to introduce to drinking water.
• Never use driftwood from tidal water, which is loaded with salt.
• Coconut shell and fruit stones make excellent charcoal if you happen to have them, better than most wood.

Making it

Charcoal is wood heated hot with the air kept out, so it pyrolyses rather than burns. You want black, not grey. Grey means ash, which means it burned, which means you have made a poor alkaline powder rather than charcoal.

The simple method, from an ordinary fire:

1. Let a wood fire burn down to a good bed of glowing embers.
2. Pull out the pieces that are glowing black rather than white-ashed.
3. Drop them into a metal tin or bucket and put the lid on immediately, cutting off the air.
4. Leave sealed until completely cold, several hours at least, ideally overnight.
5. What comes out is charcoal. Discard anything grey, powdery or crumbling to ash.

The tin method, which gives far better and more consistent charcoal:

1. Pack a metal tin (a paint tin, biscuit tin or ammunition box) with dry hardwood in finger-sized pieces.
2. Punch one small hole in the lid, about 3 to 5 mm.
3. Put the closed tin into a fire.
4. Smoke and then flame will jet from the hole. That is the wood gas driving off, and it is the process working.
5. When the jetting stops, the pyrolysis is finished. Take the tin out.
6. Plug the hole immediately, with a stick, a stone or a dab of mud, and let it cool completely before opening.

That last step matters. Charcoal at several hundred degrees will reignite the instant it meets air, and opening a hot tin gets you a face full of flame and a tin of ash instead of charcoal.

Crushing and grading

Charcoal works by surface area, so crushing it matters as much as making it.

1. Crush it between two stones, in a bag with a mallet, or in a mortar.
2. Aim for coarse grit, roughly 2 to 5 mm. Think coarse sand to small gravel.
3. Sieve out the dust. Fine powder packs down, blocks flow, and washes straight through into your water as black grit.
4. Rinse the crushed charcoal in clean water until the rinse runs clear. This takes several changes and gets rid of the remaining fines.

Too fine and it clogs and passes dust. Too coarse and water runs past without contact. Coarse grit, well rinsed, is the target.

Keep it dry and sealed until use. Charcoal adsorbs whatever is around it, including smells from the shed, and arrives at the filter partly exhausted if stored open.

Using it, and replacing it

Where it goes in the stack: always after the sand and before disinfection.

A typical improvised stack, top to bottom: cloth, then fine sand, then coarse sand, then charcoal, then gravel. Sand does the physical work; by the time water reaches the charcoal it is already clear, so the charcoal spends itself on taste and chemistry rather than being blinded by silt.

Contact time is what matters. A thin scatter of charcoal that water rushes past does almost nothing. Aim for a layer at least 5 to 10 cm deep and a slow flow. If the water comes through fast, it has not been treated.

Replace it far sooner than you would think. Home-made charcoal has limited capacity and there is no visible sign when it is exhausted. It does not change colour or texture. It simply stops working, and the water starts tasting of pond again.

Practical rules:

• Replace every few weeks in steady use, or sooner.
• Replace it as soon as taste or smell starts coming through, which is your only real indicator.
• Replace it after any incident where you ran unusually dirty water through it.
• Never try to wash and reuse exhausted charcoal. Burn it, compost it or dig it into the garden, where it is genuinely good for the soil, and make fresh.

How to know when it is spent

You cannot, by looking, and the page has said so twice without saying what to do about it. Here is what to do about it.

The problem stated plainly: charcoal does not change colour, texture, weight or smell as it exhausts. There is no home test. The only symptom is that the taste and smell it was removing start coming back, and by the time you notice that it has been doing nothing for a while.

So the answer is a schedule and a label, not an inspection.

Contact time is what you are buying, and it is the thing most improvised filters get wrong. Adsorption is not instant: water has to sit against the carbon surface long enough for the molecules to attach. A thin scatter that water rushes past does almost nothing however good the charcoal is.

• Deeper bed, slower flow. Those are the two levers and they are the same lever.
• If it comes through fast, it has not been treated. A gravity system that produces a trickle is working correctly.
• A deeper bed also lasts longer, because the exhaustion front moves down through it gradually rather than the whole thing giving up at once.

Store made charcoal dry and sealed. Charcoal adsorbs whatever is around it, so a bag left open in a shed arrives at the filter already partly spent on shed air. A sealed tin or a zip bag, kept dry, and it keeps indefinitely.

And keep the reason for the whole page in view. This layer is there to make water people will actually drink, and to take some load off the stages after it. It is not a safety barrier. The safety barrier is the boiling or the dosing that comes after, every single time.