Anyway, I could make'm taller, shorter, longer, etc. But they were still all beginning to look alike. It wasn't until I began to try to make a building lower without decreasing the width that I thought about changing the roof pitch. Nothing wrong with a gentler pitch. Here in the sunny South roofs are generally less steep....
Less snow and all that! And a low profile for hurricanes, too.

The easiest way to go was to cut a standard JS block on the diagonal. So I did and the resulting pitch looked good, so I decided to make a mold of it so that I could mass produce them a lot easier than using a hacksaw to cut my first trial concrete blocks....

As long as I was at it I thought that I would let you look over my shoulder and see a mold being made. The products that I use are simply handy to find in the Raleigh area. There are lots of good mold making compounds on the market.

To start I used Durham's Rock Hard Putty to make my original castings from a standard JigStone mold. The Durham's sets up in a couple of hours, but remains moist for a day or two. I poured the mold when I got to my shop, by lunch they were ready to be popped out.

Using a small steel straight edge and an Xacto knife I cut gently through the first
1/16" of detail on the face of each casting.  The blade was angled  to cut a shallow v-shaped trough. The trough will provide enough leeway for the use of either an Xacto zona saw or a fine toothed powered jigsaw. Either will work well in the first 24-36 hours. After that the Durham's is well on the way to being "rock" hard!

Now walk away for the weekend. Let the Durham's dry all the way through.  As a 'putty' it is generally nice and dry in 60 minutes.  But our cast JS piece is about 1/2" thick and requires a longer time. The following steps can be messed up if moisture is present in these pieces. On Monday, you may gently sand the sawmarks out and smooth off the cut face.Grab your Elmer's or hot glue and stick the castings to a small piece of plywood.

I used a 6" x 8" piece from the scrap can. One side was nicely sanded  on the belt sander. The pieces were glued down and let to dry while I went back to teach classes.

The next day I spray nuked the casting/plywood  every class break with a clear gloss. I hit the plywood heavily as well as the sides of the castings. This formed a nice mencus sealing the casting to the plywood.

I used lighter sprays on the face of the casting, so to not fill in the detail.

Again, the next day, having let the unit dry overnight, I proceeded to the dam that holds in your rubber compound. I like to use good old synthetic modeling clay - reusable, non-drying, multiuse, and rather oily. Cheap, and easy to find.

You want to keep the sides of the dam about 1/4" from the castings.  Check out one of your regular JS molds to get the idea. If you make the space too large you are:

• Making the mold too rigid. It will be very hard to pop out your final castings.

• Wasting the molding compound. Money, money, money!

At this point you are ready to go on to the next step without waiting overnight!

I use "Smooth-On" Polyurethane Rubber Compounds. Specifically, # PMC - 121/30. This compound exudes an oil which acts as its own release agent when casting with gypsum plasters and concrete. Try to stay away from PMC- 121/30 DRY as it requires a release agent. Oops, one other quick step first!  Give the unit a quick mini-micro spritz of canned cooking oil. Use a cheap little paint brush or a q-tip to spread it around. If you can tilt the unit and see the grease run, you have waaaay too much.  Kleenex time! And do not add more. What the Kleenex leaves behind is plenty!

Now I quote from the "Smooth-On" technical bulletin: "After dispensing equal amounts of Parts A and B into the mixing container, mix thoroughly for 3 minutes making sure that you scrape the sides and bottom of the mixing container several times."

"For best results, pour your mixture in a single spot at the lowest point of the containment field. Let the rubber seek its level up and over the model. A uniform flow will help minimize entrapped air. The liquid rubber should level off at least 1/2" (1.3 cm) over the highest point of the model surface."

"Allow the mold to cure overnight (at least 16 hours) at room temperature (77 F/
25 C) before demolding. Do not cure rubber where temperature is less than 65 F/
18 C."

"Smooth-On" does make a water based concrete release agent called "IN & OUT". I have cast some 2 dozen of my engine house sides without any release agent. The compound in that mold still feels oily to my fingers......

As you mix and pour, the PMC121/30 looks and flows like real maple syrup on a cold morn. If you do pour from the low point then the bubbles (Very Few!) will be swept up and off the mold pieces. Any others will float to the top - believe me! I've just poured a batch as I write this to make sure that I get all the find points written down. Even tho it looks like maple syrup I can see through the 1 inch to the bottom of the pour and I see no remaining bubbles down there! If there were any, then you dig them out with the toothpicks.

Clean up is simplified if you just use throw-away cups and popsicle sticks. The Technical Bulletin does not even mention clean up...! So there!  I'm using a old plastic butter bowl and a 5 inch spatula (4/$1 at the Dollar Store).

As it is nearing the witching hour, I will let the mold set until I get home tomorrow night before I finger test it. Even then I may leave it alone until the following morning. Then I will take some more pictures.

The next afternoon the mold was ready. Getting the mold off the original is a lot trickier than popping regular castings. This is because the two pieces are glued to the plywood. It is one big assembly! Please don't use anything sharper than the SIDE of your finger! Finger, not fingerNAIL. Screwdriver blades, Xactos, and nail files are a no-no! This will be a very frustrating 5 minutes. However, once the rubbber compound tears it will continue to tear and there is no way to fix it. Be frustrated, and get over it!

A funny story to laugh at while you pry away..... My first mold was the BIG one for the side of my engine house; over 9 inches long! Instead of using clay for the dam, I had used 3/4" basswood which I had glued and sealed down! It took me about 20 minutes and some fancy words to get the, ahem.., damn thing off!

The clay dams are off faster than I can write this sentence. Experience, my dears.

Being the careful sort of guy I am, and otherwise too busy with teaching, the mold then got another 12 hours of drying. Tonight at home I have run off 2 batches of castings. They are just fine. Here is a picture of one set along with a standard set JigStone roof pitch pieces, so that you can compare the lines.

No Longer.....

Created 17 April 2001 - Finished 21 April