Several times now, I've mixed plaster of some kind, only to have it full of bubbles. This has happened using Hydrocal, and the WS Road kit. What's the secret to thoroughly mixing but not getting any bubbles?
The secret to mixing any type of hydrocal/plaster is to first place the measured water in a mixing bowl and then add the powder and spatulate to the desired consistency. When it is the correct consistency, vibrate the hydrocal/plaster mix on a table and all the bubbles will come to the surface. Then spatulate the mix again and then pour it into the mold, vibrating the mold yet again. All remaining bubbles will come to the surface. I practiced Prosthetic Dentistry for 30+ years and this is how I made bubble free casts of teeth. Never add water to the powder in the bowl, it always will be soupy as you'll add to much water every time. Have fun..... Stay cool and run steam.....
Mixing plaster takes a lot of practice. I am just a beginner at this at my HO model RR club. They have mixed 700-800 pounds with a few thousand more pounds to go. IMHO, soupy is not good, but it will still set up. Probably okay if you are making a mold. Not okay if you want to paste it vertically against a wall. Soupy will probably not have bubbles. The thicker it is, the more likely it is that bubbles will be trapped. If you do have bubbles, wait until it gets hard enough to carve and cut rocks around and through the bubble area. Remember that smooth areas of plaster will not take stain well and will be noticeably different. Carve them, too. Another one of the variables is that different consistencies of plaster mix will take stain in different ways. It is oh, so slight, but it is different. I think that most people solve this with more weathering. I will add that I do pour the water into the plaster (a dental mixing cup) and mix it with my bare hand. That way I can tell if there are any lumps in the mixture. It is soupy, but I just wait until it starts to set up some. It only takes 1-2 minutes. Get in a club and do a lot of plaster! Practice makes perfect, as with most things. You will find many, many techniques for this.
Flash: You want a thick mix of plaster. To remove any bubbles you vibrate the plaster mixing bowl on a table and spautulate the mix heavily with a plaster spatula. I used plaster/hyrocal for 30+ years in dental practice. Vibrating the plaster mix removes almost all the trapped air bubbles. You mix the plaster with a plaster spatula, not your bare hand .If it's too soupy, you've added too much water. By letting a soupy mix set up you get a very weak porous casting full of air bubbles. You can tell if plaster is mixed properly, by putting the bowl over your head and turning it upside down. If the plaster winds up in your face, it wasn't mixed properly. This was how we taught Dental students to mix plaster. Stay cool and run steam......
Several techniques will work. If the bubbles come out easily, I didn't think it was a problem. I definitely would not use plaster on a model railroad as thick as your technique. Just my opinion.
I would add that another thing to keep in mind is to mix slowly. Mixing too fast (like whipping an egg) will tend to create bubbles.
Flash: I used my technique for mixing hydrocal/plaster for all the rock work on the JJJ&E. Mixing plaster/hydrocal correctly is the same technique for many projects. The plaster water ratio has certain properties and if you mix it to a thin consistency destroys the properties of the plaster/hydrocal mix. Stay cool and run steam..... [ December 28, 2005, 11:18 AM: Message edited by: Powersteamguy1790 ]
You have to mix the plaster briskly to incorporate all the powder in the water to obtain the right consistency. If you have the right consistency to the plaster/hydrocal mix, you have plenty of working time to do what you want to do with the hydrocal/plaster mix. All bubbles can be removed by vibrating and tapping the mixing bowl on a counter top. That brings all air bubbles to the surface. When you pour the mix into a mold, dribble the mix into the mold on one side and vibrate the mold to bring any remaining air bubbles to the surface. Stay cool and run steam.....
I disagree with most of your points on this PSG. It does not have to be mixed briskly; slow and steady will do it best. Mixing with your hands is best. You can feel the heat as it starts to set. Too much water delays the curing time and makes the coloring different. The plaster in the mold should be wet when you press it against the layout. This will allow you to carve in between the molds and thus let the stain enter the plaster. Also, it makes each rock different although it may have come from the same mold. (Smooth plaster will not stain; it will have to be painted.) I have only seen cast hard, dry plaster used for stone walls; it has to be cut on a band saw to fit into position on the layout. I have not used hydrocal successfully. USG No. 1 Moulding Plaster is the type I have dealt with. We just have to disagree on this. There are many ways to do plaster and your way may be best for hydrocal. Everyone should try the different techniques and, if possible, the different plaster types. I will try to post pictures of some rock types made the way I have described. Also, I had an awful time with Plaster of Paris.
Deleted: Stay cool and run steam... [ December 28, 2005, 06:35 PM: Message edited by: Powersteamguy1790 ]
These rocks were made from plaster using the mixing methods and castings I have described. The first picture is an overall shot of a sandstone ridge about three feet high and 15 feet long. The whole ridge was made from a single mold 18 inches square (46 cm). The mold was repeated in different positions and then carved to make the edges match. It is stained, not painted, with artists acrylics. Four colors are used: burnt umber, raw sienna, Payne's grey, and yellow ochre. These are the only colors used (for some reason) and the colors are varied according to the type of rock being modeled. We have a geologist who tells us when the colors are right. Here is a close up of the ridge (HO scale). More of this ridge above a tunnel opening. More this weekend on the limestone quarry. This may not be the way others make rocks, but this is one very good technique that works, hand mixed plaster and all. [ December 28, 2005, 05:06 PM: Message edited by: sapacif ]
When I'm doing molds I want the plaster just thin enough to pour and flow. Also I mix slowly so as to not whip in air. Then tap the molds to release air. It also helps to wet the molds. When I am attaching plaster to plaster I wet the dry plaster good so that it will not draw out the water too quick and end up with a skim set. Many a ceramic tile setter learned the hard way trying to set dry tile into thinset mortar, only to have it sheet out months later. My terrain is now all foam base, carved out, then has a thinset plaster applied over it. I use additional thinset to attach my rock moldings.
