For high vacuum (i.e. NOT UHV), Viton gaskets are fine basically anywhere. They leak water however. If you have some odd shapes that are hard to clamp you can use any variety of epoxies (from e.g. https://www.epotek.com/)
Glass to metal is the only tough nut at the low end of high vacuum. Glue works but you will always have to worry about thermal stresses crating a leak someday.
There's a new fs pulse laser technique for directly bonding glass to stainless, but its still not quite there yet.
At my company we developed a new type of all-metal seal as drop in replacement for viton gaskets. These can get you way below where viton can, and work well for arbitrarily shaped ports. It's awesome. Still a few bugs to work out to make sure you get a tight seal before cracking the glass.
Isn't Kovar to borosilicate a well accepted method of joining glass for vacuum applications? I mostly worked with UHV, as your disclaimer excluded, so am not widely with high vacuum applications.
I'd think that there would be an issue with thermal coefficients going directly to stainless..
On a mass spectrometer I recently replaced an old-fashioned triode gauge tube and these are the same size glass as the 9-pin miniature radio tubes of the 1950's.
The base looks like an audio 12AX7 and fits the same size socket, the metal pins are supposed to be Kovar coming through the glass.
But this electron tube is not sealed at the top, it is wide open with a Kovar metal precision adapter ring fused to the glass instead, and a Viton o-ring is used to lock it into the vacuum measurement port.
As a vacuum tube, it "simply" behaves differently depending on residual trace gases present. It is connected to a circuit optimized to give calibrated vacuum readings.
No doubt I could connect it to an audio circuit instead, and get some excellent output. It's a well-made tube, only a single triode though.
I've used a feed throughs that came as CF bored out with a borosilicate stubs already attached, so that you can fabricate your own glass without having to know about the metal-glass bonding. I'm not sure how big those get if you're trying for viewport style usage.
A number of things. Metal seals, welded and vacuum brazed joints, a variety of good epoxies. Even good O-rings. And helium leak chasers, so you can actually find and correct leaks.
To get down below a certain pressure, you have to heat everything and "bake out" residual volatile materials. This precludes a wax from even being tested at the lowest pressures.
If your system is cryogenic you can absolutely still use wax and wax-like products. I sealed an aluminum can for something like 30 LHe cool downs with a mixture of glycerine and soap.
Future cryogenicists: glycerine+soap is amazing because you can wash it away with hot water before you finally take the part to get re-welded. Most epoxies/sealants can't be cleaned so easily, fouling any future welds.
Just adding one more thing, I had very good lock with Swagelok Ultra-Torr fittings, which are an o-ring seal that works on either metal or glass tubing. This is how I managed the metal to glass seal when needed. I didn't do much glasswork, though. My dad is a retired research chemist, and he did all of his own glasswork unless it was something really tricky.
If needed I could silver braze the Ultra-Torr into a larger object. It's just a bit of a puzzle to figure out what sizes of glass tubing mate with the various metal fitting sizes, because glass tubing sizing can be variable.
We also figured out how to make direct glass-to-metal joints at some point. Widely used in mass-manufactured stuff, but I think you can also buy just a glass-to-metal transition or feedthrough by itself if you're doing custom glasswork.
There are a few people who make most of the copper to borosilicate glass joints in the world. It's really hard to do, and they're quite good at it. Don't try to make your own as a cost-saving measure!
A major challenge is matching thermal expansion coefficients. Copper expands quite a lot, and borosilicate barely moves at all, so they make a joint to a glass that moves less than copper, but more than boro, then they add a little bit of another glass, and so on until they've gradually transitioned from the copper to the borosilicate. Those well-made joints can be cooled to liquid helium temperature (4 K) without issue.
Yes, but you need to be able to bond glass to the alloy (note the color shift in that housekeeper's seal), and nickel alloys like kovar tend to have a really high magnetic permeability, limiting their application in experiments that need highly controlled magnetic fields.
Indium wire is sometimes used to make gaskets, especially for cryogenic systems. You have to make a new gasket every time you open the seal, though. It's much more ductile than copper and handles vaguely like stiff clay. You can sort of sculpt it together when joining the seam.
Apiezon sells a vacuum putty (Apiezon Q) that can be used for temporary seals down to 10^-4 torr. Best I can tell it's a mixture of bentonite clay and petroleum jelly.
Way back when I maintained our high vacuum setup I routinely used Apiezon H for our manifold valves. I was routinely able to get fractions of a microtorr of vacuum (e.g 0.2 x 10^-6 torr).
There is a big selection of high vacuum greases and waxes you can buy. As you mention they are basically highly pure, very heavy hydrocarbons that have almost no vapor pressure.
Glass to metal is the only tough nut at the low end of high vacuum. Glue works but you will always have to worry about thermal stresses crating a leak someday.
There's a new fs pulse laser technique for directly bonding glass to stainless, but its still not quite there yet.
At my company we developed a new type of all-metal seal as drop in replacement for viton gaskets. These can get you way below where viton can, and work well for arbitrarily shaped ports. It's awesome. Still a few bugs to work out to make sure you get a tight seal before cracking the glass.