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Subject: pcal
I have had good luck with simulating stratified sedimentary and metamorphic rock using ceiling tiles. I also discovered that they can be turned to mush with water and blended with a spatula. The ceiling tiles can be "sealed" with an acrylic primer and loose the stratified characteristics.
I felt that they could be used to recreate igneous or volcanic rocks. I "googled up" igneous rocks using their "images" feature and got very good examples and ideas.

{Click images to enlarge)
I am attempting a volcanic intursion or insertion. An area where the magma forces itself into a sedimentary crack cools and is exposed by the elements.
Make sure you have a neat and orderly materials table.
I beat a hole in my hardshell for the intrusion. I didn't fill the hole and this makes the formation unsteady in assembly until the mess dries. On the rock face I filled the hole with Woodland Scenics plaster cloth.
Hot glue blocks of ceiling tile together to form larger blocks. Hot glue creates an almost instataneous bond. Water based glues don't work well because the porous tiles just absorb them. The tile blocks should have straight edges. Trim the blocks  to make geometric shapes.
I use a Woodland Scenics Foam Knife. It provides a long sharp flexible blade.

Make sure the hot glue is in the center of the block. The solidified hot glue doesn't carve well and  won't take paint.

Glue the blocks into place with hot glue using pictures of real rock formations as reference and guide.
Run white glue down the back of the cracks to strengthen the final assembly.
The rock formation can be done as a smooth homogeneous mass or done with cracks.  If it is done with cracks don't blend all the seams.
Put white glue on a seam or area where the tile is to be blended or the shape change. The white glue dissolves the ceiling tile and makes it mushy. When the formation is finished the white glue holds the mush together.

The Woodland Scenics Foam Knife makes an excellent spatula. Put a small dab on the knife blade and apply it to the tile.

Here is our rock face as it is slowly blended together.
More tile was added at the intrusion base and top to balance the appearance.
I like Zap acrylic primer to give the ceiling tiles a rock texture. A thick coat is applied.
The rock formations were painted with grey craft paint.
Before the grey paint dries completely highlights of earth colors: sieenas and umbras, are dry brushed onto the formation.

Before all the paint is completely dried a weak mixture of alcohol and India Ink is applied. This blends the colors and the India ink subdues the highlights.

The first attempt at igneous rocks had success.
I was unhappy with the smoothness of the rock formations. I was worried about the lack of differentiation from my sedimentary rock so I homogenized their surface too much.

I added cracks by opening some of the smoothed over seams. Some were opened too much so more ceiling tile  was used to close the gap.

The lava intrusion cracked. the paint would be difficult to  reblend so I reprimered the formations.
The reworked formations were painted with artists Gesso. No particular reason to use Gesso just that it was within arm's reach and easier to open than the ZAP.
Before the Gesso had dried thin black paint was washed over the rocks and blended with the Gesso to make a light grey. Earth tone highlights were added.
The overhang along the red line was removed and puttied up using white glue and mushy ceiling tile.
Before the paint dried the reworked rock was washed with the weak alcohol and India ink wash.
Our first attempt at ceiling tiles as igneous rocks was fairly successful. Future one's will probably improve.

Adding faux fur bushes