Gypsum Cement

Dutchelm05 · Post by **Dutchelm05** » Mon Aug 16, 2010 1:37 pm

I have been struggling in making a hologram from an object that is made out of HYDROCAL® Brand White Gypsum Cement.
Does anyone know much about this material and if it is suitable in making a hologram?
Is there a materials rating that reflects its hardness and what is the threshold for a material suitable for holography.
Also can anyone remind me of the out come of the hologram if the plate moves verses if the object moves? I know Colin posted it once and I could not find it.

I was told once that it is a good thing to pre-expose the object with laser light and allow any possible heating of the object to equalize. That seemed to work although it seemed odd that so little energy (barely a mw) could heat an object. But I can't argue with the result so far.

:wall:

Peace
Tony

dave battin · Post by **dave battin** » Mon Aug 16, 2010 1:54 pm

Is the object aged quite a bit? as i recall hydro-cal becomes quite hard, you jut need to be sure all to moisture is out of it ........... i usually give my plaster objects a nice quick bake befor i seal them w/primer.....

Colin Kaminski · Post by **Colin Kaminski** » Mon Aug 16, 2010 2:15 pm

I used Hydrocal to cast violins when I was a Luthier. It held moisture for a while. Baking would be a good idea. It survives know 200F with no cracking on a violin sized casting. I would have heated it more but that was the temperature I needed for other reasons. Is the object rocking?

This might help with troubleshooting:

http://www.holowiki.com/index.php/Troub ... _Holograms

Joe Farina · Post by **Joe Farina** » Mon Aug 16, 2010 2:32 pm

Hydrocal is a hard type of plaster (gypsum). Absolutely one of the best materials to make models for holography with. For years I used Permastone (which must be very similar to Hydrocal, if not identical to it) to make models for embossed hologram producers. Not once did I get a complaint about the choice of material. But as Dave cautioned, it should be dry. If you make the casting yourself, do not use an oven to make it dry more quickly. The plaster will break and crumble. It needs to air dry naturally, fan-forced air will make it dry more quickly. Also, it's a good idea to seal the surface after drying with 3 parts IPA to 1 part bullseye shellac (available at hardware/paint stores). This dilute mixture will soak into the plaster cast without forming an appreciable surface layer (details of the cast will not be blurred). A brush or airbrush can be used, apply enough coats so that the plaster cast just begins to show a gloss or shine. Then the cast won't easily absorb moisture from the ambient air, and it would also provide an ideal surface for painting.

Dutchelm05 · Post by **Dutchelm05** » Wed Aug 18, 2010 12:55 am

Thanks a bunch guys
I am not sure had it was cured. I will investigate that.
Always something, it never ends :wall: