This has been done for hundreds of years. Some of the "Old School" versions can still be seen in some of the scenery and effects in Masonic temple rites. One of the most famous is the "Volcano" trick. The rear of the
drop is painted first. The mountain is painted like a black and white photo negative. Solid opaque black where the "Lava" is not. Bright red and orange
aniline dye where the lava is over flowing the top and running down the sides. The sky is laid in with various streaks of glowing red and highlights on the lower edges of the clouds, the top edges of the clouds are again, opaque black. The dyes and rear painting is "set" with boiled
starch mixed in with dye and paint and the front is covered with a very thin coat of
starch "size water" to seal it. Next the front of the
drop is painted in a thin water color style as a
daylight mountain scene with blue skies and white clouds. The mountain and clouds on the front and back must align exactly. The final part of the trick is to cross
fade from a steep,
neutral color, down light
wash from the front to a VERY bright straight on, back light, often in different colors to enhance different areas of the
drop. Last but not least is a piece of board (plywood or
luan today) covering the mountain from the back. As the
effect continues, the board is slowly lowered and the look from the front is that the lava is running down the mountain in slow rivers of molten rock.