Some people scratch the engravings, pour liquids on the engravings, put light coloured sand in the grooves, apply chalk, etc. All of these things can damage the engravings. Photographing rock engravings using a remote flash is probably the least damaging method available.
The method that we use is a Speedlite wireless remote flash unit. The Speedlite is reverse mounted under a tripod approximately 10 – 30 cm above the surface of the rock and aimed to fire across the face of the rock, simulating the sun low in the sky. With a remote flash you can successfully photograph rock engravings in full sun, partial shade or full shade. This is achieved by putting the camera into manual mode and stopping down the aperture in the camera to underexpose the image or you can use a suitable ND filter with a wide aperture. A variable ND filter is very handy for this situation. This darkens everything that the camera is seeing. Set the shutter speed to equal the maximum sync speed of the flash. The light produced by the remote flash unit will then bias the ambient light and lighten the rock face while (hopefully) leaving the grooves darkened. For best effect aim the flash head across the grooves as much as possible but try to ensure that the flash is generally aligned with the sun. Any grooves that line up with the flash will likely get washed out.
If you wish to create a flash composite, every frame should be taken with the flash aimed in the same direction. If the flash orientation is not maintained then the resulting images will be difficult to stitch and the result will appear unnatural. The Speedlite is controlled by a master flash unit – we use the pop-up flash that is built into the camera. Put the pop-up flash into master mode and set the Speedlite into slave mode. The master flash activates the slave unit by firing a very weak flash that triggers the slave flash.
The height of the flash unit above the rock and the best distance from the subject will require a little experimentation to discover what best suits your equipment and may vary depending on things such as the reflectivity of the rock and the distance to the camera as well as the power output of your flash unit. If it has rained recently or water has been poured onto the rock then it may be difficult to use a remote flash due to the scattered glare from the wet surface.