How to take a century of rust off a saw plate without taking any steel with it.
Rust removal usually means abrasion or acid, and both of them remove steel. On a tool that carries an etch, a maker's stamp or a hardened edge, that is a problem: the detail you want to keep sits only a few thousandths of an inch proud of the surface you are about to sand away.
Electrolysis avoids this by attacking the rust electrically instead of physically. Set up correctly, it will take a plate that is orange from heel to toe and leave every stamped letter legible, because it never touches sound metal at all.
Rust is iron that has given up electrons to oxygen — iron oxides and oxy-hydroxides, bonded to the sound metal beneath. A low-voltage direct current run through a conductive solution pushes electrons back the other way.
The rusted tool is wired as the cathode, the negative side. A piece of scrap steel is wired as the anode, the positive side. Current flows from anode to cathode through the solution. At the tool, hydrogen is liberated at the surface and the oxide layer is reduced and physically lifted off its bond; at the scrap steel, oxygen is liberated and the anode corrodes, gradually, in the tool's place. That is the trade: the anode is consumed so the tool is not.
The solution is an electrolyte, not a cleaner. It exists to carry current, and it is not consumed in any meaningful quantity. Nothing dissolves the rust — it is lifted, and ends up as black sludge in the bottom of the tub and orange scale on the anode.
This process splits water. Hydrogen comes off the tool, oxygen comes off the anode, and that mixture is explosive across a wide range of concentrations. Run it outdoors or in a well-ventilated space, never in a sealed container or a cupboard, and keep flames, sparks and switched contacts away from the surface of the tub.
Use mild steel for the anode. Never stainless: its chromium leaches into the solution as hexavalent chromium, which is toxic, and turns a bucket of harmless washing soda into something you cannot pour down a drain.
Current will not pass through oil, wax, grease or paint, and anything masked by those will come out of the tub exactly as it went in. Wash the plate with detergent and hot water, and strip any old varnish or lacquer before starting. Handles come off: wood does not enjoy an alkaline bath, and brass saw nuts do not enjoy the current.
Warm water dissolves the soda faster, but temperature is otherwise irrelevant. Stir until it is clear. The solution is mildly alkaline — it is washing soda — so it is kind to skin but unkind to eyes, and it will leave a film on anything it dries on.
Current travels in straight lines through the solution, so only the parts of the tool that can "see" the anode will clean. A single anode on one side cleans one side. Two anodes either side, or a length of rebar bent into a U around the work, cleans both at once. Nothing metal may touch anything else metal — a bridge between anode and cathode is a dead short.
Negative (black) to the tool. Positive (red) to the scrap. Reverse them and the process runs perfectly in the wrong direction: the tool becomes the sacrificial anode and starts dissolving into the bucket, which is the one failure mode here that does permanent damage. Check twice before switching on. Within a minute of starting, a fine stream of bubbles should be rising off the tool — that is the confirmation the polarity is right.
A few amps for a few hours handles ordinary surface rust; deep scale can want overnight. There is no benefit to hurrying it with more current — high amperage mostly makes heat, sludge and hydrogen. The water will turn the colour of weak tea, a crust of orange will build on the anode, and black sludge will settle on the bottom. Lift the anode and scrub it clean halfway through if the build-up gets heavy, because that scale insulates it and the current will quietly drop off.
What comes out is black, not silver. That black layer is loosened oxide and it comes away under running water with a nylon brush or bronze wool — never steel wool, which leaves ferrous particles behind to rust in place. The bare steel underneath will flash-rust in the time it takes to make a cup of tea, so dry it immediately, warm it through with a hairdryer or a low oven to drive moisture out of the pitting, and oil it straight away.
It does not restore lost metal. Pitting that the rust ate is still pitting afterwards. What you get back is a clean, stable, chemically sound surface, and an honest view of how much damage there actually is under the orange — which is sometimes a disappointment and is always useful to know before spending an evening on a plate.
It does not remove pattern or etch. This is the whole reason to bother. Acid greys and slowly eats a surface; abrasive removes it. Electrolysis leaves a maker's etch readable when it was readable to start with.
It does not care about shape. Screw threads, files, the inside of a plane mouth — anywhere abrasive cannot reach, current can, provided the anode can see it.
Atomic hydrogen liberated at the surface can diffuse into hardened, high-carbon steel and make it brittle. In practice this is a concern for springs and hardened fasteners rather than a spring-tempered saw plate, and low current with a short run keeps it marginal. If it matters to you — a hardened plane iron, a spring — bake the part at around 200 °C for a couple of hours afterwards to drive the hydrogen back out before putting it under load.
Electrolysis gets the rust off. It does not make a plate pretty, and it does not make a saw cut. A short warm citric-acid soak afterwards lifts the last of the staining that the current left behind — but acid does remove metal, so it is a matter of minutes with an eye on it, followed by a neutralising rinse in baking-soda solution, then dry and oil within the same minute.
Polishing comes next, by hand, along the length of the plate and never across it: scratches running across a saw plate catch in the kerf. And then the part that decides whether any of this was worth doing, which is the teeth.