TJ I think you were right on with your first suggestion and after preliminary attempts it appears not to be a bug! I moved the attenuation and depth sliders for reflection and refraction to mid values. A "plop" render seems to indicate the problem is resolved but at GREAT expense in rendering time. (Expected) I probably should have mentioned that I am using the fresnel plugin from Lost Horizons for the tumbler and that does not seem to cause problems. I will mess with settings and see if I can pick up the rendering speed and report back if you wish. Thanks: again once again the forums experts come through! I was ready to ditch the project!