I’ve been making a bunch of rock castings over the past couple of days basically busy work until my track order arrives and it never fails either I mix way more or not enough Hydrocal for the job I am working on. So being as I couldn’t find out any sort of formula other then the standard mix on the box of Hydrocal I figured why not go to the source and here is what the nice folks at Woodland Scenics replied so I figured I would pass it along.
note: I am guess the same formula could apply if your using plaster of Paris or some similar casting compound
As far as calculating Hydrocal needs, you can use the following formula:
Fill rock mold with water, then pour the water into a measuring cup. Measure the amount of water and divide by two. Times that amount by 2.5 to find the amount of Lightweight Hydrocal you will need.
For this example, our rock mold holds 4 oz of water.
4 oz divided by 2 = 2 oz
2 oz times 2.5 oz = 5 oz
Add 5 oz of Lightweight Hydrocal to 2 oz of water. Then, follow package instructions.
I just line up the molds in order of which ones I want the most and pour until I run out.
Next pour I change the order. I have never even tried to make it come out even.
Hi!
I too have been pouring rock molds the last three days. I haven’t done that since the early '90s when I built my previous layout. Once the good folks on the Forum reminded me to use soapy water for a release agent, I was on my way.
I had an open 8 lb box of plaster of paris, and made batches smaller than I needed and filled the molds that took up all the plaster. Then I mixed a second batch, and a third. After a day to set (I had put some vinegar in the plaster to delay set time), I removed the molds and recast another set.
While there may be a formula for determining how much to mix, IMHO there are just way too many variables to worry about that. In summary, I believe you are much better off in making smaller batches as then you will use all the plaster (have less waste), and not have to worry about using it quickly so it won’t set up in the bowl.
By the way, I bought a set of three cheap plastic food containers from Wallyworld for about $4, and with a rubber spatula the process went really well. When you are done with a container, let it set awhile and then just flex it and all the residue breaks free - and it is ready for use again!
Not to sound corny, but there is something soothing about working with plaster…
ENJOY,
Mobilman44
Me too.
I took some time to make up a couple of new molds last week, and to fix up an old one. I have a home-made mold for my subway platforms, and I plan to use it for my above-ground station as well. The mold was just latex, and I could never get a good flat casting without a few dips, so I added some plastic mesh backing (the stuff you put in gardens to control weeds) and that worked very well. I also bought a single package of Brawa cobblestone sheet, and made up a mold from that. Finally, I made a mold from my college class ring, that will be used as a sign above a seedy waterfront bar.
It’s been a while since I did much casting, and I was down to the bottom of a big jar of Hydrocal I bought a while back. The castings were terrible. They took forever to harden, and when I removed them from the molds they cracked and crumbled. I decided the Hydrocal was too old, and had likely picked up moisture, even though I always kept the lid on. I opened a new jar, and the next batch of castings were much better. No one has ever mentioned that Hydrocal has a shelf life, but from experience I guess it’s true.
Mr. B
I asked the folks at W/S if it had a shelf life and they told me no I completely agree with you in that it does . I keep my boxes even though sealed in large zip lock plastic bags. I too have been doing some experimenting making latex molds and have found cheese cloth to be a good material for strengthening molds. On one particular mold of a retaining wall being as it didn’t have a lot of thickness to it I used styrene strips about 3/8" wide around the perimeter of the mold but not right up to the edge. this way it gives it some support and still is flexible enough to come away from the casting once the hydrocal has cured.
Dental stone makes a very strong casting, but it cannot be stained like Hydrocal, it has to be painted. It is heavier and stronger than Hydrocal.