A full-faced gas mask has two features that prevent us from inhaling potentially toxic contaminants in the air. Then it has to be sealed against the face, preferably to skin (so no beards), and then, if needed, have glass eye protection. It's the filter that causes the issues...
- The first is a particle filter which removes any bacteria/dust in the inhaled air. There is essentially no chemistry involved here, just a physical barrier that forms a field between the toxins and the face. When a particle is inhaled, it hits the fibers on the filter and becomes tangled before it ever has the chance to reach the nose or mouth and move down to the lungs. This is very replicatable in medieval times. They had the ability to weave silk etc. to very fine levels, and simply having the material be dampened increases its effectiveness considerably.
Asbestos could be used which would aid this as well, as they were using it in Roman times for various items, but alas as we know, it tends to causes a lot of issues in the long run, especially if it was being breathed through. The crazy thing is even the Romans knew that asbestos causes lung damage, but no other material came close to what was needed at the time, and even now, asbestos still beats almost anything else, which is why it is still used in nuclear reactors etc., but that's a tangent... back on track.
- The second element is based on a chemical process called adsorption, and removes toxic molecules like the nerve gas sarin. Through adsorption, a solid or a liquid can trap particles on its surface — analogous to the way a cigarette filter reduces the amount of toxins a person inhales when smoking.
To filter out harmful chemicals, most gas mask filters are made with activated charcoal, or oxidized charcoal. When charcoal is activated with oxygen, it becomes ripped with tons of "sticky" holes in each molecular structure just like chicken wire. Any toxins that pass through the charcoal become bonded to these holes, and are prevented from moving into the gas mask. while there are other chemicals used nowadays, activated charcoal is the best solution for most toxic gases.
So - could a medieval society make activated charcoal?
Rough process for making activated carbon
- Make charcoal.
- Powder the charcoal.
- Make a 25% solution (by weight) of calcium chloride.
- Make a paste with the calcium chloride solution and your powdered charcoal.
- Spread the paste to dry.
- Rinse with clean water.
- Bake at 225 degrees F for 30 minutes.
This is the stumbling block, specifically chlorine. It is rarely found by itself in nature. According to google, chlorine was discovered and first isolated by Carl Wilhelm Scheele in 1774, and calcium chloride was first discovered in 1808, long after the time frame you're wanting.
So, would anything else do?
Well... Activation of charcoal is really a word to describe the process of driving condensed smoke from charcoal’s pores, and this can be done with a variety of chemicals, its just that calcium chloride gives the best results, so replacing it with even water and repeating the process several times would activate it a bit, but you'd still be breathing in smoke as the charcoal released it over time (not ideal) - you want to be able to get rid of as much smoke as possible. Coconut oil is filled with calcium but lacks the fat/cream that cows milk would wreck the whole process. So all you need is a coconut in a medieval setting. Perhaps it could have been carried by a swallow? Not a European swallow, obviously, and then again, African swallows are non-migratory... oops, another tangent.
But then, you did say with modern knowledge, so.
Making calcium chloride (wow this is getting long)
- Place the limestone in the beaker. Use enough limestone the fill the beaker about 1/4 full.
- Add a 1/4 of a beaker of hydrochloric acid to the limestone.
- The hydrochloric acid will start to bubble as it dissolves the limestone. Gently swirl the beaker to mix the contents and make sure the reaction goes to completion. If all of the limestone dissolves, add a little bit more.
- Once the solution stops bubbling, filter off the solids by pouring the solution through the filter paper into the second beaker. The calcium chloride is dissolved in the filtered solution.
- Use the hot plate to gently heat the second beaker containing the calcium chloride solution. The solid left after the water evaporates is solid calcium chloride.
So the big one here is hydrochloric acid, which, according to google again, was discovered by the alchemist Jabir ibn Hayyan around the year 800 AD!!! That's really good.
So we finally got there. Would it have been possible in our timeline, without knowledge and understanding? No.
But with modern understanding but only medieval materials - YES
might take a while and it would be very expensive to get all the materials together, but... YES!