(Providing you screen your material first, a "puffer" dry washer may be just what the doctor ordered at this location.)
I'm pleased at the amount of interest in this series of posts and the insightful comments and questions. I hope I can live up to your expectations in that regard. California miner Dan T. asked me to touch on the concept of "dead air" with dry washers so that's where I'll end up in this particular post.
First, let's review one type of dry washer commonly used by small-scale miners:
Bellows or "Puffer" Type
The bellows or "puffer" dry washer is by far the oldest and most common type of dry washer out there. I can't say exactly when it was first used in dry placer gold recovery, but I'm sure it goes back quite a way in time. A simple "puffer" was what I used for the bulk of my dry placering and it's one of those simple but efficient tools that works well for most dry washing scenarios.
(Schematic of a bellows or "puffer" type dry washer with a canvas feeder.)
Common "puffers" use a bellows set up to send bursts of air up through a removable riffle tray that's typically lined underneath with cloth or linen. One thing to note here is that the riffle tray must be removable so you can dump the concentrates into a 5-gallon bucket or other receptacle. On my old "puffers" the riffle tray was held in place by four brass snap fasteners and it took only a minute or two to unsnap them, dump the concentrates out, and snap the tray back in place. Going forward here, a simple cam and pulley system drives the bellows up and down sending puffs of air up through the linen or cloth which, in turn, act much like water does in a sluice box. The bellows can be operated by a hand crank, a wooden handle (often called a "slap" handle), a lanyard, or it can be motorized either by a gas engine or a battery.
My "puffer" was a simple wooden design that was motor driven using an old Briggs and Stratton lawn mower engine mounted on a custom metal stand. However, I've also used "slap" handle, hand crank, and lanyard-driven "puffers" in my day. I'll tell you right here and now that they require a lot more effort and sweat than a motorized version and if you go the non-motorized route with a "puffer" I recommend the hand-cranked version. Or better yet, a "puffer" than can be operated either by motor or hand...these do exist or can be designed and built as such. One last thing here that may answer a lingering question or two about sampling dry placers...although you won't find them for sale by manufacturers, I've seen many small, do-it-yourself sampling dry washers in my time. Some of these were small enough to stuff into a backpack and most were operated using a "slap" handle.
Make Sure it's Sturdy
I'm not too crazy about the hopper and feed tray set up in the schematic above and here's why. One of the down sides of using a heavy duck canvas feeder is that the canvas usually frays or develops tears or worn spots under heavy use. This is also true of most canvas bellows designs. My "puffer" employed a canvas bellows and yes, I had to replace the canvas periodically. I've also seen car upholstery fabric and even leather used for the bellows and I suspect many old timers used whatever material they had at hand that did the job. My "puffer" hopper box and feed tray were made of sturdy wood topped by 1/2 inch, heavy duty metal screen. The angle of the hopper and the feed tray could be easily adjusted by hand in just a few seconds to increase or decrease the flow of material down onto the riffle tray. The dry washer in the schematic looks a bit awkward to me...but I'm biased toward "puffers" like those I used for many years and it doesn't mean the one above won't work like a champ (and it probably does).
Whatever the case, it's vitally important that your "puffer" is sturdy and durable (they take a lot of punishment in desert placers), and is properly set up and stabilized so the pulley belts don't fly off or the entire unit doesn't break away from you and go dancing across the desert floor when you fire the motor up. (Yes, I'm exaggerating a bit but if you've ever dry washed with a gas-driven unit you know what I'm talking about). The angle of the hopper box and the riffle tray are crucial to recovering gold efficiently from dry material as are the location of the air openings or holes in the upper board or seal of the bellows configuration. If you decide to build your own "puffer" at some point the size, location, and design of these air holes is very important.
As an aside, I'll readily admit I'm not very mechanically inclined nor good as a "do-it-yourselfer," so my "puffers" were custom designed and constructed by a well-known and highly respected small-scale miner who really had the "puffer" dry washer concept down. I have to hang my head in shame here for another reason...my wife's sister (who has never mined per se) took a set of dry washer plans and with minimal help from her husband made one of the most beautiful wooden "puffers" I've ever seen. It was a gas-driven model and the craftsmanship she displayed in making it was something to behold...my hat's off to you Barbara (and a nod to Jim as well). Finally, forgive me if I've left anything out in the above or failed to address things as detailed as possible...I haven't dry washed in a while now and my memory is fading bit by bit as each year passes. So feel free to pitch in with your own observations, comments, or corrections.
My personal understanding of the concept of "dead air" as it relates to dry washers is best expressed by comparing it to the hydraulic eddy currents in a properly configured and set up sluice box in a stream. In a water-driven sluice box you'll notice that the heavies (black sands, gold, lead, etc.) tend to congregate on the down-stream side of the riffles. This is due to eddies that form in a sort of "dead zone" behind the riffles when sluice is set up and running properly. Another way to view this idea is how large rocks or boulders act as gold traps on their down stream sides in a wet placer. With a dry washer however, the riffles stop the heavier materials on the FRONT side (up stream side) of the riffles, not the down stream side. It's a similar concept, but reversed.
("Puffer" dry washer powered by a gas motor. Image courtesy the New 49ers.)
This concept applies to a "puffer" and other dry washers in general. In a properly designed, configured, and running dry washer I would offer that there should be pockets of "dead air" on the immediate up slope side of the riffles that act as invisible eddies so that the heavies and gold get trapped there and not blown or puffed out and down the riffle tray with the lighter material that is typically meant to be washed or worked down and away as fine tailings. I've seen many dry washers use quarter round wood sections as riffles...the steep angle of the quarter round was on the up slope end...the semi-steep angle of the riffle tray requires this otherwise everything is likely to go down the tray. The "dead air" pockets should hold the heavier materials here (as well as the riffles and their angle). That's my version of "dead air" anyway. If you think I'm remiss on that topic, again...feel free to pitch in again since I'm no longer out there dry washing and my real-time expertise is fading as fast as my memory.
Setting up a "puffer" (or any other dry washer for that matter) to operate at peak gold-trapping efficiency can be a trying experience, especially for mining "newbies." It takes a lot of trial-and-error fidgeting, adjusting, and fine tuning to get a dry washer (especially a motorized version) running the way it should and at peak gold-grabbing efficiency. However, with time and experience you'll work out the kinks and be able to set up and run material with a minimum of frustration and lost mining time. I've used all sorts of mining gear in my 35+ years out there and I have a very special fondness for dry washers (especially "puffers") and working desert or dry placers. I can't really explain why that is, but part of that feeling is based on the elegant simplicity of how a "puffer" works and the beauty of our western and southwestern deserts.
(c) Jim Rocha (J.R.) 2013
Questions? E-mail me at firstname.lastname@example.org