I just bought a small "lot" of pens of many different makers, one of which was a nice green and silver Parker 21 in fantastic shape.

The ink sac is still very pliable, very little scratches or anything.
Writes very well, even compared to my 51.
I paid next to nothing for it, so it was a happy surprise.

Anyway, anyway, I'm getting away from my point...

I was wondering what the ink sac is made of in the 21's?
It's quite rubbery, but it's clear, so I was wondering if it was some kind of rubber or is it a kind of plastic?
I was just curious about it, in case I do need to get it replaced in the future.

Also, would I have to replace it with another clear one, or can you use a standard opaque one?

I believe it's a vinyl similar if not precisely the same as in the aero "51" filler, which happily means unlikely replacement. I think I've seen the basic, unprotected sac over at Wood Bin, if need arises.

oof. well, that's mildly distressing, but not too horrible if the need arises I guess, thanks!

Its called pliglass and it is extremely durable, but it stains with ink, so if you see a truly transparent verion, it probably is uninked. The only failures I've seen is if somebody causes the sac some trauma.

Methinks Skybird has hit it on the head. Pliglass. And yeah, from what I've seen, it can stain quite badly.

well, when I got it, it had a little bit of dried blue ink in it,
which i flushed out thoroughly with water, partially to make sure it didn't leak,
but the sac was still clear and unstained then, so I don't know what the deal is.

I've got standard black Quink in it, which works fine so far, trying it out, just because it's a Parker,
but I think I'll move on to a nicer blue-black or even some deep blue ink soon, which I rather like.

They can stain, but a lightly used one, especially with some reasonable ink like Quink or Skrip can still be almost entirely clear. The 21 I regularly use is only very slightly bluish.

Is there a replacement for it?>

Yes, the same sac as the Parker 51... you can get them from Tryphon

The original 21 sacs, like the "51" aerometrics, appear to be a PVC flexible plastic of some sort. Except for staining, they will likely outlast you or I (particularly me!).

Parker Superchrome seems to stain rather badly -- I've got several stained that lovely turquoise that Superchrome came in, along with black. No effect on function, just impossible to see what's in there.

I find a fine nib 21 will write about 20 pages for me on a single fill -- rather nice.

Peter