i would probably try loading a more viscous material so it doesn't mix and just ruin your good material, although i would test it with something that isn't going to hurt to much if it fails first.
also try filling the tubing with the expensive material or test stuff, then switching the syringe once you have enough in the tubing to a slightly more viscous paste. not sure how much your planning to use, but filling the syringes with 2 different materials is almost guaranteed mixing and wasting some.
a clay slurry would work perfect here, just make sure its wet enough since if its too thick it will have lots of trouble extruding.