Bellows Sealing

[Lucas]

So I've made a pair of single action bellows (ala the top couple image on this page http://www.anvilfire.com/FAQs/the_forge_2.php but bigger). I used canvas instead of leather. I experienced tremendous losses of air prior to sealing (as would be expected). I have sealing the canvas with boiled linseed oil. When the oil was still wet it sealed really well, but now that it is curing I've noticed that there is still some air loss thru the canvas. Does anybody else have experience using canvas for bellows? What did you use to seal it? Should I re-coat with linseed oil? Should I try roofing tar? Any ideas or thoughts would be greatly appreciated. It is my plan to build a ground-forge and use these as an air source. I'll try to get pictures of my actual bellows.

[pkrankow]

What type of canvass did you use? Did you get some duck from the fabric store, or from an old Carhart jacket, or did you use a canvass painter's drop cloth, or is it from some other source. The grade of a canvass varies greatly and some are better than others. You will need a high level of suppleness from the final product, so you may want to experiment on scrap that you have leftover.

Take several samples of canvass and try different treatments on these swatches. See if they are supple and air tight after they are cured. I would try exterior house paint as one myself. Since I already have some of the 1/3 wax, 1/3 turps, 1/3 linseed oil mixed up I would try that too.

Adding another layer of canvass with a different material (poly tarp?) in between is also an option if you have the canvass.

Phil

[dablacksmith]

ive always used leather for my bellows ... the leather i used wasnt much more than good grade canvas costs (a 3 ft double bellows useing cheap pigskin was under 50 $) ive heard of people useing canvas but never seen it . ide make sure it was a good tent type of canvas that had water repelent in it . vivatex is one brand and it has fire retardant also the lynseed oil finish will catch fire easily (not so good around forge) . on fresh canvas you might try tompsons water seal (not any more fireproof but a good sealant for canvas)good luck!

[Lucas]

I used duck canvas. Painter's tarps seemed a bit "airy" to me. I like the idea of outdoor paint seems easier than roofing tar. And less likely for me to make a mess that I can't clean up that my wife will be less than pleased with than with tar. Was the only historical option leather or was other stuff used at times?

[southernforge]

Rubber tool dip, in spray or can. Result similar to "bellows cloth" for musical instruments.

[ThomasPowers]

Tar and boiled linseed oil both are designed to get HARD. Bellows need to be flexible! (They are also quite flammable, perhaps not the best around fire either!)

I built a double lunged bellows back in the early 1980's using scrap canvas that was used for making wind shields for oil drilling rigs---heavily treated "tarpaulin" material. It was free from a shop that supplied the oil drillers. Is free cheaper? After 20+ years the canvass was going strong but the wood was degrading rapidly from being left outside in central Ohio winters and I gave it away when I moved out west.

So I advise you to go looking for tarpaulin material and at the source not priced up through several middlemen.

As to historicity of using canvas I recall reading a reference to canvas being used for bellows in Biringuccio's "Pirotechnia" circa 1540 A.D.

[Lucas]

Ah, perhaps the linseed oil was a mistake then. I thought that oil cloth was made from canvas coated with linseed oil and left to cure. The solvents in boiled linseed oil are highly flammable, however, once the cross-linking of the oil occurs it shouldn't be more flammable than wood would it? But I figure you all know more than I do about these things. My day job is as a chemist (think chemistry, not pharmacist) so flammable can be a bit of a relative term to me. Do you usually fireproof canvas bellows like with sodium borate or something?

[ThomasPowers]

The heavy tarpaulin for oil field use seems to have been well treated against flame when I got it. At least in the 20+ years I had and used it there were no burn throughs.

Oilcloth was probably originally made with real boiled linseed oil rather than heavy metal doped driers linseed oil---don't you just hate it when they change stuff but keep the same name!

as to differences in flammability, the thinness of cloth compared to wood in the system can make things easier to start.

I'm familiar with your field---my sister is a Chemist; used to work for IBM. (My degrees are in Geology and Computer Science)

[Dave Budd]

on the only occasion that I've used cloth for bellows (normally I use leather) I coated it with PVA glue and it was good and airtight (also flexible and non-flammable).

I once waterproofed a canvas using an old recipe ('for making new or reproofing lorry tarps') made by mixing boiled linseed and tallow. it was flexible and watertight, but golly it was heavy! :lol:

[pkrankow]

on the only occasion that I've used cloth for bellows (normally I use leather) I coated it with PVA glue and it was good and airtight (also flexible and non-flammable).

I once waterproofed a canvas using an old recipe ('for making new or reproofing lorry tarps') made by mixing boiled linseed and tallow. it was flexible and watertight, but golly it was heavy!

So that's yellow or white elmer's glue? How reduced is it?

Phil

[Lucas]

I think both white glue and yellow glue are both PVA based, as you can use either to make "silly putty" with borax/liquid starch. Bookbinding uses a lot of PVA now too. Also, for those interested I did a test with a piece of 1 day cured boiled linseed oil on canvas vs. bare canvas. Interesting results. The canvas acts like a wick for the boiled linseed oil coating (which tends to burn like plastic more than oil, as would be expected from a crosslink) but apparently enough of the linseed oil breaks down to form a flammable oil to keep the flame going like a candle (the canvas chars, but does not burn away until the linseed oil is consumed near that area). When the flame is put out the smolder does not propogate and when you blow on it the smolder goes out. With regular canvas fire does not propagate as well but once blown out the smoldering continues and when you blow on the smoldering it spreads and tends to continue to burn. Also the canvas is consume when it burns by itself not "charcloth" remains. I'm going to coat a piece of canvas with wood glue and do the same test.

Just performed the test with a PVA glue coated sample. In a transverse flame test the PVA coated canvas self extinguished in 10 second and the smolder does not propogate extinguishes in 30 seconds. 3 replicate tests, times averaged, measuring tool: counting in my head. :)

[rthibeau]

When I rebuilt my double lung bellows, I used canvas....an old army tarp which was heavy and looked oiled. It works great. So maybe check for Army surplus.

[Don A]

A lot of folks use red barn paint for waterproofing canvas to use for primitive shelters. I think it is either latex or oil based, which might be flexible enough for a bellows.

It might be a good idea to paint the outside and let it dry, then finish the inside with a coat of borax... mix with water, paint it on, let it dry. This would proabably help as a burn retardant.

This is what I'm planning for mine. I will, however, test the paint on some scrap first.

Good luck, and keep us posted.

Don

[Lucas]

Well, I am sure I will be doing a rebuild of a smaller set of more efficient bellows before too long, but as of right now. I had already stapled the canvas to the whole thing (one of several issues). Anyway, I have sealed the whole thing with Elmer's white glue. (fyi, you DO NOT need a gallon to do this, I have 3/4 of it still left, lol) Anyway it is a little stiff, but still flexible enough to work and the canvas is sealed very well.

[Rich Hale]

I have had a couple of canvas tents and they leaked water when new. the instructions with the latest and most expensive one said to set it up prior to using for tent and turn the hose on it so the material will shrink. First wetting it did leak.. several more wet and dry cycles and it is water tight now. I applied a seam sealer on all seams and it is good to go. The newest is now near 30 years old. Certainly not the same as a bellows...or maybe it is..?

[edge9001]

back in the days of canvas tents they were waterproofed in one way, shrink the fabric. this tightens up the holes in the weave. I actually heard some people boiled the canvas and then let it airdry in order to waterproof them. I;m not too sure how truthful that is though.

[pkrankow]

back in the days of canvas tents they were waterproofed in one way, shrink the fabric. this tightens up the holes in the weave. I actually heard some people boiled the canvas and then let it airdry in order to waterproof them. I;m not too sure how truthful that is though.

I have no doubt about the success that would have since cotton shrinks quite a bit, especially if it has not been preshrunk.

Certain types of felt are made as a rather loose knit or weave and then boiled to shrink to a fraction of the original size.

Phil

[Lucas]

That's actually the technique I've used to make felted berets, as well as how my younger sister (who weighs half what I do) ended up with a couple of my felted sweaters. lol.