How Do I Seal Silver Reaction Tube?

Looked at cold pressure welding?

(confess I haven't, but it just came to mind).

If the silver is absolutely clean, the crimping alone should provide a weld. 50 years ago I made vacuum tight seals in 1/4" copper tubing simply by crimping. The tool was a pair of bolt cutters, where the normally sharp cutting edges were ground to about a 3/16" radius, so you were essentially squeezing the tubing between two 3/8" rods. The items being sealed had just been removed from a vacuum furnace, where they were first sealed by melting a glass tube, so the insides were immaculately clean. Since the glass could break, it was a temporary seal, and the crimping was the permanent seal. I still have a sample ( the outside half of the crimp with attached glass seal and intermediate expansion ring) and its still gleaming inside, although the outside has oxidized from being handled many times.

Now I have to admit that these were to hold vacuum, not pressure, so try it with caution!

if you find that the solder is required, use that same process on both ends. It takes a lot less heat to solder than to weld! Even that may be impossible with part of the silver in water.

Finally, investigate friction stir welding.

The crimping you describe is still used in the vacuum industry. I'm aware of it being used for aerospace applications where the units are subject to high levels of vibration & stress without failing. As you say, this is to seal vacuum so anything greater than 1 bar would need some testing.

Try Master Bond ... www.masterbond.com

They make numerous types of sealants; maybe they can help.

Cheers! DZ

How about using a compression fitting cap. i don't know what your exact intent is, but compression fittings are reusable except for the ferruls and there are many that are extream high pressure (10,00 psi, 690 bar).

I am just learning to work silver with fire so I don't have much relevant personal experience. Jewelers do have some special products and techniques for soldering or repairing items without removing set gems which are heat sensitive, which might be applicable.

Several "heat shield" methods/products are discussed by participants in this thread, and helpful technical pointers (going in focused, hot and fast)

This is another professional overview about heat shielding gems during soldering: several methods discussed and laser welders or fusion welders are identified as the high end (expensive but ideal) solution.
http://www.ganoksin.com/borisat/nenam/soldering-with-gems.htm

Low tech solutions varying from wet paper towels to steel ball bearings or pieces of fire brick in your water bath would give additional heat protection to the acid-container end of your test tube. In my limited experience fusing silver, I found that the position of the item vis a vis the brick makes a huge difference to the amount of heat required - firebrick sucks up heat like crazy.

Sterling is more difficult to fuse (more sensitive to getting "enough heat") than pure silver which is what I mainly work with. You definitely need to use a flux to get a fusion weld with sterling, or the surface will be difficult to flow. A simple paste of borax and water brushed on is the 'least-noxious' flux alternative afaik.

A second point if you are trying to get a fusion weld with the least amount of heat, is to bend the crimp end that you're trying to weld, at right angles to the body of the tube so that it is in a different plane of focus from the object. This allows you to focus the necessary heat on that edge exactly. A titanium soldering pick is a handy tool for finding exactly where the hottest point of your flame is situated - stick it into the area where you're trying to heat and by its glowing you can immediately localize the hot spot. It won't stick to your work.

If I were trying to do your job, I would probably try to set up using a suitable piece of firebrick and drilling a hole where the tube could lay inside it horizontally, and make a water container for the brick and all that allowed the weld-intended end to stick out the side. I'd crimp the end to be welded, bent upwards 90 degrees from the body, use flux on the edge if it's sterling, and apply the flame downwards to the crimp edge hot and fast. If Milo was my boss, what would he say about this setup?

Jewelers use low power density gas torches because they need to visually monitor work progress. Higher power density (measured in watts/cm*3) is available from electric arcs, e-beams, and lasers. However, the highest power density and lowest cost heating method is resistance heating. A consequence of high power density heating is that there is less heat conducted into the work, that needs to be absorbed by auxiliary heat sinks. Silver can be difficult to heat with resistance heating because of its outstanding thermal and electrical conductivity. In these cases, refractory metal electrodes such as tungsten or molybdenum are used.