Hi Mike,
I mix the soda and van **** together; be aware when doing test pieces that it can take overnight before the soda has had the full darkening effect it's going to. This mixture can make a very nice colour if you get most of the darkening from the soda and add just a little Van **** for some tone. I would mix up the soda and use that alone on the test pieces to find a strength that gives you almost all the darkening you want, then add the van **** to get the final result. NB occasionally, using chemicals (ie the soda) to colour timber can give a blotchy look. Your test pieces, if they're from the same batch of timber, should give you an indication as to whether this is likely to happen or not.
A good trick in applying water stains and dyes is to dampen the timber first. This helps in avoiding problems with overlapping brush strokes and allows you to apply extra stain in streaks along the grain to avoid an unnatural, too-uniform look. Don't let the timber dry out in patches (a mister bottle helps with this) until you have finished applying the stain. If you use this approach you will need a darker mixture than you would otherwise use as it will be diluted by the water already on the surface. Lots of test pieces and lots of experimentation is the key!
For the most authentic aged look the timber needs to be oxidised (which is what happens to timber left to age naturally). Nitric acid is the best way to do this, but its nasty, nasty stuff. Often it goes too far with the darkening and then you have to pull it back with bleach (hydrogen peroxide) which is also nasty, nasty stuff, although less nasty than the nitric. This gives an authentic aged look.
The amonia treatment — ie a dish of amonia (again, nasty stuff!) in a sealed plastic tent with the furniture for a day or two — will give an authentic 'old' look on oak. This only works on timber with a lot of tanin in it. I believe that this is the same basic effect as the washing soda (which also reacts with the tanin) but I may be wrong about this.
Don't think about trying any of these if you don't know what you're doing with dangerous chemicals - the danger is real.... I use them in antique restoration but I wouldn't use them on the scale required for new work (except perhaps the amonia if I was doing reproduction oak furniture....). For me the washing soda and Van **** is a reasonable and practical compromise.
Regarding the silvered look (which you tend to get on aged exterior timber, not really on indoor furniture ) you can simulate it very well by dissolving wire wool in vinegar for a few days, then applying the liquid, perhaps bleaching afterwards.
Cheers,
Marcus