sluice

A sluice is a wooden or metal trough, typically 12 to 20 feet long and 12 to 18 inches wide, through which a controlled stream of water carries sediment and ore. The sluice sits at a slight gradient, usually 1 inch to 1 foot of drop, allowing water to flow continuously downslope. Gold and heavy minerals settle to the bottom while lighter material washes out the end, making it one of the oldest and most reliable methods for concentrating placer deposits.

The interior of a working sluice is ribbed or fitted with riffles, wooden slats or cleats mounted perpendicular to the water flow. These riffles trap heavy particles and create pockets of slower water where gold settles out. A miner feeds auriferous material (pay dirt mixed with water) into the upper end, and the force of the flowing water does the separation work. The operator controls water volume with a headgate, a sliding or hinged barrier upstream that adjusts flow rate to suit the texture and density of the ore being worked.

Variants include the cradle or rocker box, a smaller portable sluice that can be tilted and rocked by hand, useful for prospecting or remote claims. Long sluices are sometimes built in series (called sluice boxes run in tandem) so material passes through multiple stages of concentration. Some operations use corrugated metal liners instead of wood, which wears longer and costs more upfront. Water supply is critical: a sluice needs enough volume and head pressure to keep material in motion without sweeping away the finest particles containing gold.

Why it works, why it fails

Sluices exploit the density difference between gold (specific gravity 19.3) and common rock and sand (2.6 to 2.7). Water moving at the right speed lifts light material out of the trough while gold drops fast and lodges behind the riffles. Failure usually stems from incorrect gradient (too steep washes out the gold; too flat lets it settle unevenly), fouled riffles (clay or fine silt clogs the pockets), or insufficient water flow. Miners must also contend with fine gold, particles smaller than mesh 200, which can escape if the sluice is not designed to trap them.

The sluice remains the standard tool in small-scale and artisanal gold mining because it requires no power, uses only gravity and water, and can process large volumes of material for modest capital cost. Larger commercial operations often use sluices alongside shaking tables and spirals, but the sluice is the first stage where rough ore is broken down into manageable fractions. Its name comes from the gate (sluice gate) that controls water release, from Middle Dutch sluis.