Break the Ceiling Tile into Small Jaggedy Pieces about 1-1/2 to 2 inches long.
If the Ceiling Tile does not look like this your pieces are too uniform. Make sure the white surface is down when you glue them in place. Next time I will try Hot gluing two layers together to get a thicker rock strata
Hot Glue the pieces to the Plaster and Paper towel base. I am using Floor Leveler, which is a much more available version of Hydrocal Plaster. I would mix layers of Pine Bark and broken Homosote when I do my next rockface to vary the strata texture. Putting the tile on an angle makes a more dramatic rockface.
Apply the rock color. I applied my base scenery color which matches the paver sand I use for ground base.
More base color applications. The tiles smooth out with too many applications of water based material. This can be good or bad depending upon the look you want to achieve.
I wanted the strata layers to be less regular so I poked and prodded them and filled them with plaster, drywall mud and uncolored Sculptamold. I found later the best filler is a 50/50 mixture of latex paint and Sculptamold. I then tried various colors on the layers. The September 2004 Model Railroader has an excellent article by Tony Koester on rocks.
This is the "finished" rockfaces. I still think they need more work. Make sure you have the final lighting for the rockface to shadow and highlight. That is what I want to do next.
This is ceiling tile on the horizontal at my loads-in/loads out cut, it needs some more work.
Here the tiles have been smoothed and integrated and brushed with a 50/50 mixture of base latex paint and Sculptamold.