Mats and Underlay
Different riffle systems perform better when matched with certain types of matting.
Some confusion exists between real expanded metal mesh and the diamond-patterned punched plate material, which is very flat and one-dimensional. Actual expanded metal mesh is two-dimensional and if you see the two products side by side they are easy to differentiate between.
Flat diamond punched plate serves no purpose in a sluice or high-banker unless it’s being used as some form of grizzly for gravel classification, and if you’re using it over miner’s moss in a sluice it isn’t contributing to the collection of heavy materials in any way, but many so-called ‘economy’ boxes are sold in this configuration and the material needs to replaced with real mesh in order for the sluice to perform properly.
Composite mats can be made by layering several types of fabrics and then sewing or stapling them together at periodic intervals. One method used successfully consists of layered ‘crochet mesh’ over course-weave Scotch-Brite abrasive pads which are especially effective on fines.
Small modular range hood filters made from very fine layered expanded aluminum mesh are also perfect for sluice boxes.
With respect to mats in a sluice, the possible combinations of various materials, used either alone or in combinations of multiple layers, have endless possibilities.
When using any type of matting, it is an absolute must to have some way of stopping the flow of captured materials once they enter the mat due to the continual water flow that runs inside or beneath the matting. The more ‘open’ the weave of the material, the more important it is to control the flow. Using a mat with an impervious backing will not stop this slow migration of at least portions of the heavy materials that will eventually exit the sluice. This flow in the very bottom of the sluice, or actually inside the matrix air space of material like miners moss, is known as the ‘underflow’ , and in miner’s moss it can be significant depending on what weave and thickness of material you are using. There are many materials out there similar to Nomad fabric that are not nearly as dense so you have to be careful about what you select.
The simplest way of minimizing the effects of this under-flow is to install small (1/16 to 3/32-inch diameter) rods running transversely about every 6-inches on center in the bottom of the sluice. These rods can be welded in place or just secured with J-B Weld or some similar epoxy. The rods basically act like small ‘water-stops’ or ‘gasket-bars’ between the matting when it is compressed by pressure on the riffle assembly sidebars. In effect you’re creating small pockets beneath and within the matting that act almost like tertiary collection troughs. The rods are extremely effective in slowing both the quantity and the velocity of water running through the mat material. Ideally the rods should be placed in a position that is directly under a riffle. Even a small piece of wire, like a section of coat hanger, under the riffle has been proven to reduce the underflow of fine heavy materials.
Alternatively, 1/8 or 1/4” V-groove matting can be used on the bottom of the sluice under other types of mats and the ridges in the material behave much like the rods in creating mini water-stops as the upper mat is compressed by the riffles and sidebars.
A very interesting fact is that in tests that were conducted in a bare aluminum sluice with no matting of any kind being used, as long as there was sufficient water flow to keep the concentration vortexes operating, very little material was lost. What is interesting is that it points out the fact that matting actually doesn’t aid in concentrating and consolidating material but acts instead more like an entrapment element that helps to prevent particles from be scoured during periods when the water flow changes or is interrupted, disrupted by a fresh load of gravel, or is stopped completely at shut-down.
This helps to remind us that it’s the water flow combined with the settling rate of heavy materials, aided by centrifugal vortexes, if using riffles or expanded metal mesh, that collects and concentrates the heavy materials. There seems to be a popular, and ongoing, misconception that matting acts like a filter or sponge that collects the heavy materials as they are washed down the sluice run and this effect is indeed present but to an extremely minor degree. It is the expanded metal mesh or riffles that actually provide the turbulence that concentrates the heavy particles and forces them down into the mat.
Keep this in mind as you’re designing your own sluice, as even the best underlay will not make up for a poor box design or improper water velocity.