The original vacuum brewer was invented by Robert Napier, a Scots Marine engineer who obviously, looking at the design, spent too much time in the chemistry lab. These "Naperian" brewers have recently been revived by a Belgian company, Coffee4You, at www.coffee4you.com. The basic principle extends to all vacuum brewers, though, where water heated in one vessel travels through a tube to a second vessel containing ground coffee. The heated water and coffee mix for a period, then the heat is removed from the first vessel. A vacuum is formed by the absence of steam in this first vessel, and the brewed coffee is sucked back into it along the tube, and filtered on the way.

The beauty of this process is that the coffee is brewed by extended contact with water at exactly the right brewing temperature, the temperature is maintained throughout the process, and then the coffee is immediately separated from the grounds.

Of course, things HAVE changed a bit since Napier's day, and modern vacuum brewers are generally made of glass rather than brass. They usually consist of a jug at the base, with a glass funnel that fits into the neck of the jug, which seals with a rubber gasket. A variety of different filters are used in the neck of the funnel, cloth disks, paper, ceramic, plastic plates and glass rods, but all the filters have a single purpose. This is to allow the free passage of liquid (hot water up, coffee down) but keep the grounds up in the funnel.

I've personally collected a number of different models over the years, as you can see from the photos below. The coffee from a syphon can best be described as "crystal clear", with great purity of flavour and aroma and no bitterness added by the brewing process. Any faults in the coffee flavour are also shown up with great clarity, so syphon users tend to gravitate to the best beans they can find.

 

These brewers are still manufactured around the world, especially in Taiwan and Japan, but also by Cona in Britain and Bodum in Denmark. The Bodum Santos coffee syphons ( the "correct" name for the glass brewers) are the least expensive, but old Sunbeam, Silex, Cory, Farberware etc. versions can still be bought at various junk shops and internet auction sites. I use and sell the Bodum Santos brewers, and below is my "how to" for them.

The major problems of the Bodum syphons are that they only come in one size, the handle of the jug is as un-ergonomic as possible, and the plastic plate filter can be more prone to blockage than the glass stick types. I also use Cona syphons, but these are an expensive indulgence. The Conas come in a variety of sizes, with the glass stick type filter, but I can buy 4 Bodums for the price of a same size Cona!

 I have yet to see a well designed and supported "mass market" range of syphons, although Cona could have gained this position years ago; sadly, it appears that they have lost interest.

 The reason I use Coffee Syphons to brew in is that they are simply the best! No other brewer that I have seen can give the same purity of flavor and lack of bitterness. I believe that this is due to the exquisite temperature control, since the coffee brews at about 2ºC below boiling point, without ever actually boiling. This model can brew a minimum of two cups and a maximum of eight. It works well on either gas or electric hotplates (I use gas at home) as long as it can sit level. If the hotplate doesn't allow this then you need a wire mat, since when all the coffee is up the top the unit is top heavy and can fall over (major catastrophe!)

HOW DO THEY WORK?

The 2 main components are a funnel and a bowl. The filter , a grooved plastic plate, sits in the funnel and acts as a valve. It must be hooked onto the funnel by the spring before brewing. It will allow water (or Brewed Coffee) to move freely but traps the grounds in the funnel.
I fill the bowl with (usually) boiling water and measure out medium grind coffee into the funnel, after placing the funnel into the bowl. I normally use 10g, one rounded dessertspoon, of ground coffee per 200 ml cup. The scoop supplied is 7g. The minimum you can brew is 400ml.
I then place the assembled syphon on to a hotplate - gas or electric is O.K., - and this is where the interesting bit starts to happen. The build up of steam in the lower bowl forces the water up into the funnel, where it mixes with the ground coffee. A quick stir wets the grounds into the water, and a small amount of water left behind in the bowl keeps the steam coming and the temperature constant. Brewing continues for 2 minutes ( it can go longer but you don't get any more flavor) and I then take the syphon off the hotplate.
With no more steam being produced, a vacuum forms in the bowl, which sucks the brewed coffee down through the filter. If it doesn't suck down properly, immediately place the syphon back on the heat. Remove it as soon as the water bubbles up again without stirring, this dislodges any coffee blocking it.
Finally the funnel is removed and placed in a stand, and the bowl, filled with delicious hot coffee, is ready to pour. Since Bodum does not supply stands, I use a 400ml straight sided acrylic tumbler, which works well.