[matthewh (OP)]

Ruomei has asked us a slippery question:

Why does soapstone darken with use? The more used area is darker than the less used area, without oiling, why is that?

Help us shed some light on the topic!

[Bored chemist]

"Used" for what?

[evan_au]

Are you thinking of a whetstone (very hard, used for sharpening knives) or soapstone (very soft, used for carving or moulding).
See: https://en.wikipedia.org/wiki/Sharpening_stone
https://en.wikipedia.org/wiki/Soapstone

[Colin2B]

I had assumed the carving soapstone which I have used. It’s a form of talc and surprisingly used for kitcken worktops.
I had assumed the darkening was natural hand oil on the surface, but someone said it is oxidisation. Can’t think why it would oxidise if it’s talc, but I can’t find info on that.

[chiralSPO]

Oils adhering to the surface makes much more sense to me than oxidation.

Oil would darken the stone by reducing the amount of light that reflects of the many interfaces in a rough surface. And the stone is likely to come into contact with hand oils where it gets handled, but not in the little crevices are parts that are rarely touched.

I don't know how soapstone would get oxidized (it's mostly oxides already), and I don't know why oxidation would darken it, if it could be oxidized. Perhaps there could be traces of transition metals in low oxidation states that would darken when oxidized, but I could just as easily expect it to get lighter on oxidation... Also, if exposure to the atmosphere were the mechanism, it would be obvious because it would extend evenly across all of the surface that is exposed to the air (and may or may not extend to parts of thee surface that are not, depending on how good the seal is with whatever is keeping the air out).

[Colin2B]

Thanks @chiralSPO
I looked at the formula for talc and couldn’t see anything, but my chemisty isn’t good enough to spot any but the most obvious reactions.
I know there are impurities in the stone, but I wouldn’t think they are in great enough quantity.
General oil and grease make more sense to me.

[Bored chemist]

My best guess is that oils are darkening it, but I can't rule out oxidation of (in particular) Fe(II) to Fe(III).

And, until the OP gets back and explains the circumstances a bit, nobody knows what the answer is.

[ruomei]

IMG_0600.jpg (58.22 kB . 800x533 - viewed 2635 times)OP here.
Question clarification: I am referring to soapstone countertop. The "architectural" type of soapstone is considerably harder than the ones for sculpture, and often used as countertop/sink/fireplace. A common maintenance to achieve a (often preferred) darkened look is to oil it with mineral oil, with high frequency initially, then gradually dropped frequency. Putting on mineral oil will darken the soapstone immediately, but I assume it is just an optical effect, and it can be wiped off easily. I would like to understand how the multiple oiling process can achieve the more permanent darkened look. It happens to countertop that is not oiled too.  At more "used" area, which is touched more, by hand, water, grease, and probably got cleaned up more frequently too, the stone will gradually darken by itself. An example for such darkening can be found in the attached picture. The sink area and the power outlet area is much darker than the rest.
The most plausible explanation I found is oil polymerization on houzz (sorry, can't external link to credit the original poster and forum, search for "do you have Pietra Cardosa or Soapstone that you keep grey?" on houzz discussions forum).

It (Oil) cannot get absorbed by the non-porous stone, so most of it eventually evaporates. (This is why, for intentional oilings with mineral oil, you must repeat your initial applications.) However, some of the oil polymerizes. This increases the molecular weight, and slows the evaporation.

Anyone have better theories, or can dig deeper in the oil polymerization idea?

[ruomei]

Sorry, I claimed to be the OP because this question was sent through email to Naked Scientist, hoping to be answered in the program, disappointingly didn't make it, and then probably posted to the forum by a staff member(?).