When you purchase a new iron it will need to be tinned. By that I mean applying flux to warm iron tip and then rolling tip in solder. It needs a base of solder on it to work properly and to be maintained.
While soldering you can clean tip of flux residue (black) by dragging soldering iron tip over damp sponge of brass tip cleaner. This will help keep iron in good working order.
Metals oxidize when exposed to air and hot metal can oxidize quicker if not tended properly. This can lead to a hard blackish-brown crusty type of build up that won't wipe off. This build up interferes with the heat transfer from the iron tip to the solder and will greatly impact your work.
After trying both the sponge and flux to clean the tip of your soldering iron, sal ammoniac would be the next step if the others didn't work.
Sal ammoniac for cleaning your stained glass soldering iron tip comes in a solid block form similar to that of a bar of soap. It is hazardous so please read all safety tips included in or on the packaging. To use it, simply rub the hot iron tip across the surface in a bit of a scrubbing motion. It should work fairly quickly but sometimes the buildup is a bit stubborn to come off. Once the whole tip is clean and shiny, immediately add solder to it and it will flow smoothly around the whole tip.
Every week I loosen the collar that keeps the Weller 100 tip on and clean. If you do not keep the iron clean the flux overspray off so tip and collar do not rust.
Do not use a file, metal grinder or emery cloth to sand or scrape the iron tips clean. This will only damage the plating on the iron tip and ruin it. Try the cleaning methods mentioned above and if these cleaning procedures do not work, just get a replacement iron tip.
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