After looking at the photos and hearing your comments, I decided to do a faux marble finish on the Blue and White room's fireplace mantelpiece.
And I took pictures :)
1. Reference photo When you're doing any kind of faux finish (and that includes many things we do in the miniature world, from making mini food out of polymer clay to faking bricks on the inside of a fireplace), it's best to have a reference photo, or several.
Here's mine, which you may have seen in an early post -- it's from Chatsworth House:
I like the idea of a grey marble mantel to cut some of the "prettiness" of the gold wall colour.
2. Base colour Most marbles begin with a pale base colour, either white or cream. This one certainly could have, but I decided to start with a deep base colour, namely the tester pot of Amherst Gray last seen as the stone coloured test wall behind the blue and white plates. It's just about perfect! I painted both the hearth and mantle with two coats. (NB: This is a plaster mantel. At a couple of small points (including one of the caryatid's noses) I moved too quickly with my water-based paints and sort of "melted" tiny bits of the plaster! Probably best to let things dry thoroughly between coats, instead of going at it like a mad thing all in one gasp, the way I did :)
3. Two colours of glaze. In order to create this look, the paints you put on top of the base coat need three properties: they need to be somewhat transparent, they need to be "smooshable" i.e. moveable, and they need to have a longish open time, i.e. they can't dry too quickly. This is especially important because I'm using ordinary artist's acrylic paints -- they dry quickly and hard.
There are many products for acrylic paints which make a great base for a glaze -- I used a matte medium. Any acrylic medium that extends the drying time will work beautifully.
Working very quickly, paint on diagonal wiggly lines of the two colours (I used raw umber and the wall colour, a pale gold). I painted the raw umber first, and you can see in the photo below it's drying already -- not good! (You can also see that all this detail is going to be a pain in the arse to marbellize :))
This one is better, on the top of the mantle :) Smooshier, wetter.
Now take a small piece of cotton cloth or a bit of sponge, and smoosh the two colours gently together. You'll still see some of the base coat, and you should still be able to make out the two coloured glazes. Refer to your reference photo.
3. Veining. Once you've got the glazes to your liking, it's time to add veining, one of the most characteristic features of marble. Most marbles have white veins, some marbles are veined in black. My example has white veining, but also areas of a burnt sienna sort of warm red colour. I perhaps should have put that in as a glaze, but I'll do it with a small brush at this point, instead :) Use a very, very fine brush.
In general, veining follows the lines of the coloured glazes, with little side trips :)
4. Final details and varnish. Go back over your piece, referring to the photo. Look at it from a distance and close up. Does it please you? You can always use your base paint to tone things down or to "erase" areas that aren't pleasing and start again :)
Generally the rule of thumb for miniatures is "don't make large things glossy", because too-glossy woodwork or furniture disturbs the illusion of scale. If anything, we generally use Dull-Kote or other means to try to keep surface finishes from being too reflective. Marble is an exception -- it's not marble unless it has a suggestion of surface polish. I used a satin varnish for the fireplace.
And here it is, in the room, this morning!
Now I'm eyeing all those other white fireplaces in the house .... Look out! It's a Marble Attack!
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