The right-angle riffle is generally made with some 90” iron angles. They are the favorite in large scale operations because of their endurance. The iron angles come in different sizes but the most used is the 1 ½ x 1 ½ inches. Although one can use larger ones or smaller ones when processing large or small volumes of materials in the different sluice box sizes.
The right-angle riffle is also a riffle for concentrates. The best results are obtained by locating riffles equidistant from each other, being the same distance or greater than the width of the angle. These riffles will concentrate better when slightly inclined forward. In the case of a 1 ½ x 1 ½ inches angle the ideal inclination is around an eighth of an inch. Better results are also obtained in some situations when inclined riffles at double distance from each other are set. This is becoming very popular among miners nowadays.
The third type of riffles that is very much seen in sluice boxes nowadays is that of expansive metal, the turned-up kind. The expansive metal is used with a turn-up side pointing in the direction that the flux of water follows across the box. So a similar back eddy is created behind the turn-up points. This causes the expansive metal to concentrate and recover gold very well.
The expanded metal when used I proper conditions is an excellent recovery system especially of fine gold. Better results are obtained with the recovery systems in which more volume of materials has been pre-screened out, and which soft water flux passes through the box.
The reason for this is that riffles are so superficial that it only needs a light stream of water to keep the expanded metal concentrated properly. To much speed in the flux of water tends to leave the expansive metal clean from a part of its concentrates of heavier material, including gold.
A good place to use expansive metal as a riffle system is in a small sluice box or in a bi sluice where all the big materials have already been screened out so that a moderate speed flux is all you need in the box to move the material over the riffles. In this case you will find that expansive metals recovered gold fairly well.
Currently, most sluice boxes have some kind of rough and porous mat under the riffle system. Its purpose is to give gold something on the bottoms edge where it can stay stranded. For this different kinds of indoor/outdoor carpets can be used. The grassy type works pretty well. Some miners prefer using various layers of burlap, or “miner’s cloth”, which is a heavier type of burlap.
Something we have to remember about any set of riffles, especially of the three previously mentioned types of riffles, is that the entire surface of the bottom part of each riffle must be firmly secured against the bottom matting of the sluice box during the operation. If one leaves a space between the mat and the riffle, a portion of the back Eddie effect that happens behind the riffles will be lost. This can cause part of the action of concentrating to stop or diminish, and for that gold recovery can be affected.
Sometimes expansive metal is used in conjunction with other types of riffles. In the case the expansive metal is put under the upper set of riffles, as a set of Hungarians for example. It is generally a good idea to make sure the underlying mat is thick enough so as to stick out around it and not through the expanded metal and fill the space between the metal and the lower edge of the underlying riffles, because if not some of the concentrating action in the upper set of riffles can be lost. This means there will be a loss in fine gold recovery.