Blanket Mold pg 2
Becoming familiar with the tools and materials of mold making.
Now to make a sweet little blanket to keep our pineapple cozy. We will make the blanket out of clay and place it on the model. It is a fairly simple straight forward process. You're going to love it. The water based clay may be obtained from the Compleat Sculptor.
The clay I am using is a gray water based clay. Not that it matters but I have found that the gray colored clays do not stain the plaster as much as the red and orange clays do. Start forming the blanket by placing a roll or lump of clay between two 3/8 " thick boards. The consistency of your clay is critical in order to be able to roll it out. If it's too wet you'll have a big sticky mess on your hands. It's like sticking you rolling pin in a pot of tar. If it's too dry it will crack and crumble when you try to roll it. So here's what you do. Start with a rolling pin that you borrow from the kitchen and roll out slabs of clay about a foot long and 4 to 6 inches wide. Because you are using 3/8" thick spacer boards every slab will be exactly 3/8" thick. The boards take all the fight and guesswork out of what could be a bear of a job. For me the hardest part of the job is getting the rolling pin back into the pantry without getting caught.
Press the slabs down tightly to conform to the shape of the model . Be careful not to press the clay too hard on the high spots of the model. This will cause the clay to thin out. When you make your mold, you might have a place so thin you can see light through.
Add consecutive slabs of clay joining them together where they meet until the entire model is covered.
Steels are thin flexible steel shaping and smoothing tools. A set of professional steels can be purchased from Chavant CO. and the Compleat Sculptor ( See our suppliers page.) These are a great help in smoothing out rough clay.
At this point some engineering of a pour hole and vent must be considered. Air must be allowed to escape and the liquid urethane must be allowed to flow in smoothly
Lets take a close look at the design of our model.
At the highest parts of the model we need to install a means of letting the air rise up and escape. We also will need a place to pour in the urethane.
At the highest elevation points we will place cone shaped risers made of clay. The yellow area represents the clay. You will notice in the diagram above that some parts have been made thicker to let the air flow up and out. If the clay followed the contour of the model in those places it would trap the air in pockets ruining the mold. This is the quick and easy way it uses more material but sometimes materials are cheaper than labor. This is the method I will be using.
Another method to let the air out of the pockets would be to place straws in the clay at the high points. In the diagram above, yellow represents the clay and the straws, white represents the plaster shell. Notice only one clay cone is necessary to act as a pour hole. With this method the clay can conform more closely to the model. It takes a little more time and effort but you use less urethane for the mold.
Now the smoothing out process begins. A natural sponge called an elephant ear sponge is the traditional mold making sponge for smoothing clay.
We have added clay riser tubes and started smoothing the surface. Just look at these forms. Is this a thing of beauty or what.
Notice the Saran wrap sticking out from under the clay. Just as it should.
Should it come to pass that you are unable to obtain one such elephant ear sponge; a small oval synthetic sponge cut in half, as shown below although not traditional, will suffice. Wet the sponge and rub the surface of the water clay to remove any rough texture. You will soon develop the skill to create a fine even surface. The object is to get the surface glassy smooth. Holes and rough ridges on the surface will keep the backup shell from registering properly on your finished mold. The smoother you make the clay now the less work you will have to do later in the plaster stage. After wiping with the sponge, the surface might look lumpy though they are smooth lumps.
Lumps and bumps in life are inevitable
Up and down and in and out curves are fine on your mold but you do want to reduce a bumpy surface. Fortunately for your blanket mold we have a way to smooth them. Wooden tools that are specially made for smoothing and flattening clay are available. They are ergonomically designed and have the proper shaped ends for gliding over undulating clay surfaces. Would that we could all have ergonomically designed ends and glide over undulating surfaces. Life would be a dream. Oh yeah!
However back to our saga. Here's a nifty little trick. A satisfactory tool for smoothing down lumps can be made from a broomstick which you haveflattened on one end. It's not quite as elegant as the store bought'n ones but it will work.
As mentioned before, there are thin metal scrapers for smoothing. There are also rubber and plastic squeegee's for smoothing purposes also. A combination of all the tools and methods above can be used to get the desired finish. There is a knack to working and smoothing water clay in this fashion but fortunately the learning curve is a short one. There is a sense of pleasure in working with these natural clays and getting them to come out smooth and blemish free. Achieving mastery of these techniques will give you a sense of accomplishment .
Test the thickness of the clay blanket at this time. Take a paper clip and straighten it. Poke the straightened paper clip into the clay at various locations. If you run into any thin spots add the proper amount of clay there. Pay special attention to high points and sharp protrusions. The clay becomes thin over those areas. Smooth the tiny holes left by the paper clip testing.
OK your clay blanket is now all neat and pretty. If you have to leave it for any length of time, cover it with plastic wrap so the clay doesn't dry out. If the clay starts to dry, it will shrink and crack.
Next you need to cut and seal and wax 4 boards to construct a form around the model. First spray the boards with Krylon acrylic sealer or use a brush on lacquer.
Next apply Butcher's Bowling Alley Wax or a commercial mold release wax. I have been told that the Bowling Alley Wax is a better choice than the Butcher's Antique Polish. I am using Butchers Antique Polish here in the photo. I have not noticed any difference in the releasing of either of these products. They are usually available at local supermarkets.
Apply three coats, polishing each to a sheen between coats. I try to avoid using shellac on boards where urethane will be cast. You can use it if you let the shellac dry thoroughly. That would probably be a for day or two. Urethane has a pure unadulterated lust for moisture. If the urethane draws out the moisture in the shellac, it will cure with a tacky surface or not at all.
Also urethane will bond to shellac like all get out if you don't wax it well or spray mold release on it. If you are in a hurry lacquer or acrylic sealers would be better choices.
Once they have been waxed the side form boards should be installed 2" or 3" from the edge of the clay. Use screws to attach the forms. You will need to remove one of the boards and replace it in exactly the same position as part of the process. The screws will allow you to do that.
Soap 'er up
The boards have been screwed down around the edge of the base board. Also note the Saran wrap has been trimmed from around the clay blanket.
Give the clay and the boards a coat of mold soap.