The brown ring inside a well-used mug is tannin residue rather than dirt, and ordinary washing does not remove it. Several household methods do, and one common one causes permanent damage.
What the Stain Is
Tea and coffee contain tannins, which bond to surfaces and build up in microscopic imperfections in the glaze.
Detergent is designed to lift fats and food residue and does very little to tannins. That is why a mug can be genuinely clean and still look stained.
What to Avoid
Scouring pads, wire wool and cream cleaners with harsh abrasives. These remove the stain and scratch the glaze underneath.
A scratched glaze then stains faster, because the tannins have more surface to bond to. The mug is permanently worse, which is why the aggressive approach is self-defeating.
On any surface-decorated or metallic piece, abrasives take the decoration with the stain. Under-glaze artwork is protected by the glaze; anything applied over it is not. Catalogues that state which pieces carry fired-under-glaze art, of the kind at silverbuffalo.net, are worth checking before choosing a cleaning method.
What Works
- Bicarbonate of soda made into a paste, rubbed with a soft cloth
- White vinegar, left to soak for an hour
- Denture cleaning tablets, which are formulated for exactly this
- Salt and a little water as a mild abrasive
- A soak in hot water with a spoonful of bicarbonate, for stubborn cases
Bicarbonate paste is the standard answer: mildly abrasive, harmless to glaze, and effective within a minute of gentle rubbing.
Steel Tumblers
Stainless stains less visibly and does hold odour and residue, particularly at the base and around any seal.
Bicarbonate works here too. Bleach should not be used in vacuum-insulated steel, and abrasives dull a brushed finish permanently.
Where the Smell Comes From
Almost always a gasket or a seal rather than the vessel. Lids on tumblers and travel mugs harbour residue in places a rinse never reaches.
Removing the silicone seal and washing it separately resolves nearly all persistent smells, and it is the step almost nobody takes.
Plastic
The hardest to keep clean, because plastic scratches and the scratches hold both stain and odour.
Bicarbonate helps. Sunlight fades some staining, particularly from turmeric and tomato, and is worth trying before replacing something.
Beyond a certain point a scratched, stained plastic cup is due for replacement rather than restoration.
Preventing It
Rinsing immediately after use rather than letting a mug stand does more than any cleaning method. Tannins bond as the residue dries.
A mug rinsed straight away rarely develops a ring at all, which is the cheapest solution available.