Tea Stains on Mugs

It does happen. Nothing to do with salt - as a child we titrated various liquids in chemistry and the water here is nearly as soft as de-ionised water, yet we've had it happen! That's with two different dishwashers and various types of powder/liquids/tablets.

SteveW

I'm no chemist, but IIRC tannic acid extracted from oak galls forms a slightly hydrphobic gel at PH neutral which can either be made soluble in acid, or neutralised with a mild alkali, like bicarb. AFAIK this explains why either method removes tea stains.

That's not the cause of the problem.

There are 3 causes:

Abrasion if the glasses rub on each other or other items.

Hard water scale, caused by the water softener or rinse aid not working. This is on the surface and some wipes off on a finger, and it will vanish if wiped with vinegar.

Reaction with lead in the glass, which causes a milkyness which goes below the surface and normally can't be removed. Tends in effect higher quality glass, which you shouldn't wash in a dishwasher if you want to keep this from happening.

In message , Andrew Gabriel writes

Yes, the latter is what I experience, our expensive lead crystal glasses get hand washed only.

I'm wondering if the reason it affects some of our cheapo everyday glasses and seemingly not others is due to differences in the precise content of the glass?

Almost certainly. We have an old Pyrex (actually Jobling) mixing bowl that has been regularly washed in a diswasher over the years. It has become etched almost all over, but quite clearly reveals flow-lines where the initial gob of glass was pressed into the mould. The flow-lines are alternately shiny (not etched) and matt (etched). They are either the result of small compositional inhomogeneities in the original glass batch, or structural variations within the glass itself, I've no idea which, but the flow-lines are clearly visible and I can think of no other explanations.

Bingo.