Absorbent stone absorbs whatever is dissolved in the water it takes in, which includes pigment. Dealing with that is straightforward if you get to it early and considerably harder if you do not.
Why It Happens
Not a fault. It is the direct consequence of the mechanism that makes the coaster work: a network of connected pores drawing liquid in by capillary action.
Water carries dissolved colour with it, and once the water evaporates the pigment stays behind in the pore structure. Nothing has gone wrong; the material is doing what it does.
Deciding It Is Not for You
Worth being honest about. A household where red wine gets spilled and forgotten is not a household for porous stone.
Retailers who say so plainly and point at their non-porous line instead, as at thirsty-stone.net, are giving better advice than one that pretends the limitation does not exist. Stainless wipes clean completely.
The Usual Offenders
- Red wine, which is the most common by a wide margin
- Coffee and tea, particularly with sugar
- Turmeric and curry, which stain almost everything
- Beetroot and dark fruit juices
- Anything oily, which behaves differently and is harder
The Baking Soda Method
The standard approach for water-based stains and it works well within about a day.
Make a thick paste of bicarbonate of soda and water. Spread it over the mark, leave it for an hour or more, then rinse with clean water and let the coaster air dry completely, face up.
The paste draws the pigment out as it dries, using the same capillary action that put it in. Repeat once before concluding it has not worked.
For Oily Marks
Oil does not respond to water-based drawing. A paste of bicarbonate with a small amount of hydrogen peroxide, or a dedicated stone poultice, is more effective.
Leave it considerably longer, covered, and expect partial rather than complete removal.
What Not to Use
Not bleach, which can damage the stone and may not lift the mark. Not acids such as vinegar or lemon on stone generally, since they etch some types.
Not a dishwasher, which combines sustained heat and detergent and will lift the cork backing off.
And never mix cleaning products. Combining anything containing bleach with anything containing ammonia produces hazardous fumes, and no coaster is worth that.
Prevention
Wipe the face after use if something dark has been on it, and let coasters dry flat rather than stacking them damp.
A sealant can be applied to porous stone, and it reduces absorbency, which defeats the purpose. That trade is rarely worth making.
Accept Some Patina
A stone coaster used daily for years will darken. That is ageing rather than damage, and many people prefer the look of a used set to a new one.
Where uniformity matters, the printed and embossed lines hide it considerably better than bare pale stone.
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