Inspired by some plans from 1942, I made my own gas mask from a snorkel, a tin can, and some charcoal I made from coconut husks. It works great!. I varnished my boat and didn't get a headache!
I'm not wearing it now, I can smell the fumes from outside, and I AM getting a headache.

This gas mask is very similar to the successful British Small Box Respirator used in WW1. Inside the familiar cloth hood with goggles it had a nose clip, a mouthpiece and a hose to a can full of charcoal and soda lime. My canister only uses carbon as the absorbing agent since I'm not concerned about "acid gases". If you need to filter out military poisons or chemicals similar to them, add the soda lime and other ingredients.

There are many avid gasmask collectors and enthusiasts. Images and information about gas masks is very abundant online. The French collector's site gasmasks.net has an amazing pictorial database covering gas mask evolution in all the worlds armies.

In 1942 Time Magazine published the following piece:
"Homemade Gas Masks
Monday, Sep. 07, 1942

An emergency gas mask that can be made at home was demonstrated in Manhattan last week by the American Women's Voluntary Services. The necessary materials can be found in almost any house: a bathing cap, a small tin can, the transparent cover from a powder-puff box, a bit of wire net (from fly swatters), two handkerchiefs, elastic ribbon, adhesive tape, and (from the drugstore) a few ounces of activated coconut charcoal and soda lime. The principle behind the homemade mask is simple; the assembly is more difficult. The rubber cap is fitted snugly over the face and two holes are cut in it; one for the powder-puff cover (to look through), one for the tin-can respirator. The ends of the can are removed, replaced with the wire net. Inside the can go the chemicals (two parts activated charcoal, one part soda lime) wrapped in the handkerchiefs. All openings in the cap are hermetically sealed with adhesive tape. An elastic-ribbon harness holds the mask on tight. An alternative model makes use of rubber baby pants (see cut) instead of the bathing cap.

When carefully made, this improvised gas mask is effective against all known war gases. But the A.W.V.S. and the War Department are leary of inexperienced workmanship. The least carelessness in fitting the parts of the gas mask together would permit gas to enter. The A.W.V.S. recommends that all such masks be made under its supervision.

An even simpler mask is advocated by Dr. Kearney Sauer of the Los Angeles Citizens' Defense Corps: two twelve-inch squares of bed sheeting with a quarter-inch layer of baking soda between, held in even distribution by crisscross stitching. Dampened and held firmly over the face, this napkin will give temporary protection against any gas, according to Dr. Sauer but not the Army.

The simplest device of all consists only of a beer can filled with absorbent material and a clothespin to clamp on the nose. It is approved by no one except its inventor, Chemist Vernon Bowers of Baltimore, Ohio.
"

Step 1: 1942 - Popular Science

"Rather than go without any protection at all, the alert citizen can make gas masks for himself and his family."

So says a December 1942 Popular Science article on how to make that same type of improvised gas mask.

The following information may help you understand the instructions better.
"10/20 mesh" and "4/10 mesh" specifies the grain size of the crushed charcoal.
Sorting grains by size is called "screen classifying". To do it you put a stack of sieves on a sieve shaker with the coarse one on top. To make 10/20 you'd put a sieve with a 10 mesh (ten wires per inch) screen on top of a 20 mesh sieve. You'd pour your crushed charcoal in the top and shake the stack. The 20 mesh sieve would fill up with the good stuff, you'd dump that into your gas mask cartridge.

In other words the charcoal grains they use are between .05" and .25" diameter. I've read that that resistance to breathing while doing strenuous work is the major problem with gas masks. A mask made with such large grains might have very easy air flow.

Soda Lime ( w'pedia ) is calcium carbonate (lime) reacted with aqueous sodium hydroxide (lye). It absorbs carbon dioxide among other things.

Carbon tetrachloride is dry cleaning fluid, very common in those days when synthetic fibers were rare and many people wore clothing such as suits that would be harmed by laundering in water.
Star says "CCl4 note that carbon tetra chloride is carcinogenic like pasta sauce is red, and you probably don't want it in your house or on your hands or anywhere"

The text of the article continues: "
This type of mask, designed by Dr.Simon L.Ruskin of New York City, is intended to protect the wearer only against the common, known gases used in chemical warfare. It is useless against smoke, illuminating gas, and carbon monoxide (automobile exhaust).
How such a mask is made is shown in the accompanying photographs. Use a heavy bathing cap, not a thin one. Make joints airtight so that the wearer can breathe only through the canister.
The physical filtering agent in the canister is activated granular charcoal, which can be obtained at drug stores and wholesale drug houses. To test it for activity (the ability to take up and hold gases), place a small amount, such as a tenth of a gram, in the palm of the hand and pour on it five drops of carbon tetrachloride.The charcoal should become warm.
If not, it's unsafe. Either one half 10/20 mesh and one half 4/10 mesh, or the coarser charcoal alone, may be employed.
With it mix one half as much coarse-mesh soda lime (sodium calcium hydrate), also available at drug stores. Pack the canister solidly. No air must enter without passing through the charcoal. Do not forget to seal both ends of the canister with tape or heavy waxed paper. If left unsealed, the charcoal will absorb moisture from the air until it is saturated, and the mask will be useless. Unseal the canister only when the mask is to be put on for protection against gas. Once the mask has been used, the canister must be refilled with fresh charcoal and soda lime. "

Step 2: "Common Materials" Back in 1942

In case you can't read the little labels, they read: From left to right:
Canisters (they appear to be ordinary tin cans)
Bathing cap
Elastic
Handkerchiefs
Fly Swatters
Charcoal and Soda Lime
Powder-Puff Box
Adhesive Tape

They sure had some odd stuff laying around back then.
Dirt roads and horses caused a lot of fly swatting and nose blowing.

Step 3: 1942 Mask Step-by-step

The powder-puff box has a clear lid, this is the window the user looks through.
The bathing cap is the mask that fits about the face.
There is no valve. The breath goes in and out through the filter.
The charcoal/soda-lime mixture is wrapped in two layers of hanky and packed into the can.
The pieces of screen are taped over the ends of the can and any excess cloth cut off.
Then the can shoved into a hole in the mask and sealed on with tape.

Step 4: Start a Fire

Here's my version of the gas mask. I need to filter paint fumes and sanding dust, not phosgene, so I didn't bother adding soda lime to my filter.

Start a fire. That's the first step in making charcoal.
Coconut shell charcoal is standard for gas masks. I wanted to try coconut husk charcoal.
So I husked some of the driest, lightest coconuts I could find.
Three of them were starting to sprout. Yay! I ate one of the sprouted coconuts.

I started a fire. It was horrible. I couldn't get the coconut husks to burn properly. It smoked a lot and wanted to smolder. In Mexico the husks had flamed out and burned down to a nice bed of coals in minutes. I guess they were a lot drier than the husks here in Maui.

I tried all my fire making tricks. I ripped the husks into strips and stacked them like a log cabin.
I added tinder of various kinds. I finally dumped paint thinner on the burning pile. That flamed for a while but then it went back to smoldering. I finally gave up and decided to let it smoke. Eventually the heat would make some of the stuff in the middle into charcoal. I piled a whole lot more husks on and after an hour or so it looked like the final picture, ready for my coal mining operation.

It's a calm day and the smoke followed me around everywhere.
I needed a gas mask.

Step 5: Eat Canned Pears

I needed a can.
So I had to eat some canned pears. Life is hard.

Then I cut an 'X' in the remaining end of the can to make a hole for the snorkel

Step 6: Fit Snorkel to Can

While that nasty fire smokes up the neighborhood, get back to tinsmithing.
Bend up the flaps of the 'X' and fit the can to the end of the snorkel.

Step 7: Affix the Can to the Snorkel

Make sure the tube doesn't protrude all the way into the can.
Drill a hole through the can flaps and the end of the snorkel tube.
Insert a wire through the holes.
Bend the ends of the wire over.
The assembly will feel pretty secure.

Step 8: Seal the Joint Between Can and Tube

Wrap the end of the can and the joint with the tube with plastic cling-wrap and tape.
Try sucking air through the tube while the open end of the can is blocked. It shouldn't be possible.
It's very much easier to seal vacuum leaks than pressure leaks. Vacuum leaks tend to suck things into them and seal themselves.

Step 9: Quench Some Coals

I looked into my fire and saw some good coals glowing inside. So I knocked it apart, grabbed some good coals with tongs made from two sticks, and quenched them in water.

I was wondering whether my charcoal would be "activated" or not. After some reading I decided
my wasteful way of making it pretty much guaranteed it would be "activated". That means no hydrocarbons left in the pores and all the carbon being short chains with their little arms out ready to grab molecules.

Here's a 1919 blurb explaining making activated charcoal and soda lime fillers in WW1. from WAR GAS INVESTIGATIONS WAR WORK OF THE BUREAU OF MINES (google books)

Many different substances were carbonized, and the resulting charcoal tried for a gas-mask absorbent. Different kinds of wood were tried, also nut shellsincluding coconut shellslamp black, carbon black, blood, seaweed, and ivory nuts. The first coconut charcoal came from Dr. H. B. Lemon, of the University of Chicago, and by December, 1917, nut shells formed the basis of most of the charcoal used in gas masks.

In preparing highly efficient charcoal the main idea is to clean thoroughly the pores of the charcoal of any hydrocarbon residues, which greatly reduce the absorptive power. One method of cleaning is to let the charcoal cool in air so as to absorb as much oxygen as possible, and then reheat it to a comparatively low temperature. Oxidizing with steam has certain advantages, the charcoal being ground to about 8 to 14 mesh and steam treated.

Soda Lime.
Soda lime, a mixture of caustic soda and calcium hydroxide, is used in gas masks to remove acid vapors or gases from the air. A great deal of study and experiment was given to the making of a suitable product, and by December, 1917, through the efforts of the bureau, a factory, capable of producing 800 pounds a day of this material was actively at work. The soda lime finally produced was much superior to that made by the English.

Here's another version of the same story from THE CHEMICAL WARFARE SERVICE Clarence J. West (google books)

the preparation of active charcoal resolved itself into two steps: the formation of a porous, amorphous base carbon at relatively low temperatures, and the removal of the absorbed hydrocarbons from the primary carbon and the increase of its porosity. The first step involves the destructive distillation of a material (cocoanut shell was found the most suitable wood) at relatively low temperatures, in thin layers so that the deposition of inactive carbon from the cracking of hydrocarbons, would be avoided. The second step is much more difficult, and was finally accomplished by oxidation with air, steam or carbon dioxide steam, all of which were used in the manufacture of gas mask carbon.

In addition to the use of cocoanut shell (Dorsite), other sources were developed, such as anthracite coal (Bachite), and a synthetic product made by carbon manufacturing process
from lampblack, powdered coal and other suitable materials (Carbonite).

Soda Lime. Charcoal alone is not a satisfactory all-round absorbent because it has too little capacity for certain highly volatile acid gases, such as phosgene and hydrocyanic acid and also because an oxidizing agent is the best means of handling certain gases. It has, therefore, been found that the use of an alkaline oxidizing agent in combination with the charcoal is advisable. The material actually used was a soda lime containing sodium permanganate. The ratios used were 60 per cent. 6-14 mesh cocoanut shell charcoal and 40 per cent. 8-14 mesh soda lime permanganate granules. The last mixture suggested, which would have had a distinctly greater all round efficiency, was composed of 75 per cent. specially impregnated cocoanut charcoal and 25 per cent. soda lime containing no permanganate.

Step 10: Crush Some Charcoal

After quenching the coals I set them on some cardboard to drain and dry out a little. Then I dumped the water out of the pot and used it for a mortar to crush the charcoal in. I used a section of bamboo as a pestle.
Quenching had drawn a lot of water into the charcoal. It turned to wet paste as I crushed it. I mashed it til the largest grains were well under .25" in diameter. The volume decreased a lot as I crushed it. This husk charcoal wasn't very dense. I needed to grind a few batches to get enough to fill my canister.

Step 11: Dry the Crushed Charcoal

I spread the crushed charcoal paste on some newspaper and put it in the sun.
It was still wet after a day in the sun so I dumped it onto tinfoil on a pan and put it in the oven.
I tasted a mouthful of charcoal to see if it was any good. It didn't taste like a smoldering fire, which is what I was afraid of. I couldn't figure out what the taste was. Then I realized the taste was the taste of no taste at all. It was absorbing all the stuff from my mouth that could activate a taste bud. This was a non-taste I've probably never experienced before. I remembered how the old nuns in the old-folks convent at home used to eat burned toast for the charcoal. Those nuns were the best educated people in the area for a hundred years or more, so I figured they knew a thing or two. I chewed it up and swallowed that mouthful of tasteless carbon with gusto. I smiled with clean black teeth.

Step 12: Base Manifold Wad

I put a loose wad of paper towel in the base of the can. I wanted the airflow to spread out from the tube into the can without restrictions. I put a disk of folded paper towel on top of that to keep charcoal dust from going into the tube and my lungs.

Step 13: Pack the Can With Charcoal

I packed the can with charcoal til it was about as high as the longest gas mask cartridge I've seen.
The can was taller than that, so I added some wadding on top of that. to hold it in place.

Step 14: What Dissolves Aluminum?

The next day I noticed the aluminum was full of holes. Something in the charcoal had dissolved a lot of holes in it. Maybe sodium hydroxide from wet ashes? The foil had been laying on a steel cookie sheet with wet charcoal on top of it. Had I accidentally made a battery?

Step 15: End Cap

I taped a folded paper towel over the end of the can, cut off the excess, and taped it some more, just like the 1942 version. I don't know what the function of the screen was in that model. Probably it was for durability. They probably intended people to actually fight and dig through rubble while wearing those swimcap and baby pants gasmasks on their faces.

Since I'm just going to do some light varnishing, I'll stick to mere paper towels.

Step 16: Exhale Valve

This snorkel has an exhale valve so air doesn't have to get forced back out the filter. The air flow is one direction, which means there's no dead air going back and forth in the tube for CO2 to build up in. Because of this exhale valve, I could use a longer more flexible tube. That would probably make it more comfortable to wear.

Step 17: Varnishing!

I considered trying the "clothespin on the nose" to keep me from inhaling through my nose, but it turns out to not be a problem. Something about a snorkel makes a person not want to inhale through their nose. Not to mention there's a bad varnish smell to remind me if I forget.

The whole thing works great! I didn't get a headache and get all stupid from fumes. I tried shoving the tube up against a nostril and inhaling through it that way, and couldn't smell anything through it.
If I start smelling anything through it, I'll repack it with new charcoal.

The only problem was condensation dripping out of the exhale valve onto the varnish. I tied a rag over it to catch that. When I wore it as seen in the opening photo, no drips came out.

This respirator works so well I'm thrilled. I'll look for a flexible hose so I can hang the canister on my belt. It will be perfect for welding, since a welding hood won't hit the mouthpiece.

Can any chemists or welders in the audience tell me if I need to add the soda lime to filter welding fumes? Apparently manganese, manganese dioxide are in the fumes and cause harm.

Step 18: Mishaps and Improvements

I used the disk sander while my gasmask was lying nearby on the table. I should have been wearing it. When I remembered and put it on, I inhaled sawdust that had blown into the mouthpiece. It was pretty unpleasant.

One problem with mouth-breathing is that when dirt or dust gets into the mouthpiece, you huff that right into your lungs. Put a bag over the mouthpiece when you're not using it.

Later I thought it had happened again. Actually the wadding had shifted from banging the can around. I'd inhaled charcoal dust into my lungs. That's probably really good for you, but it's no fun. Much coughing. So I took the filter apart and wrapped the charcoal up just like in the 1942 instructions.