A SHORT PREVIEW FROM THE CASTING BOOK




Here is part of the chapter on brick making …

AN ADVENTURE IN BRICK MAKING

        Making a brick is an expedient way to learn by doing. If you take the time to do this little project you will definitely have a frame of reference and a feel for the details that follow in this book. Bricks are almost as simple to make as gypsum or cement ice cube shapes. In fact, if you are really pressed for time with the basics here, just go ahead and use a plastic ice cube tray as your mold. It will make quick but unusable bricks.
        If you are a modeler, you can have fun making little bricks. If you are a promotional advertiser, you can make full size, or even giant, bricks with custom company names and logos casted into the tops in relief. You could make story bricks, on the idea of story buttons, for use as doorstops or paperweights. Or maybe you want to make little red brick dog houses or bird houses. Or garden marker bricks. I hope you get the idea here; a brick does not have to be just a brick, if you cast it yourself.
        If you are willing to make a full size, usable brick, then the thing to do is to make a mold out of wood. For a full size brick, take some 1x3 pine and eight drywall screws. Cut two pieces about three and a half inches long and two pieces about ten inches long. Try to cut all of the edges as square as you can. A mitre box, chop saw, or table saw would do an adequate job. Paint the wood with linseed oil or motor oil before assembly. The oil will seal the wood against water and act as a release agent. See figure 1.

figure 1

        Pressing some modeling clay into all the corners is a good idea. This makes smooth corners and edges on your castings. Smooth, slightly rounded edges are stronger and neater. Your castings won’t require touch up. Also, crumbling of the edges of your castings from wear and weather is less likely to occur when the edges are slightly rounded. Smooth the clay into the corners with your finger.
        If you prefer, wax heated in combination with some olive oil can be used instead of modeling clay. If you want this to be a production mold, auto body putty could be used instead of wax or clay.
        You will want to lay the mold on a flat surface, such as a table top or a piece of plywood. You may want to put a piece of wax paper under the mold for easy parting of the casting. You could also paint all mold surfaces using a soft paint brush loaded with petroleum jelly or light grease. Petroleum jelly and light grease are excellent mold release agents. They will stick to the verticle walls of your mold and make separation of your cured casting much easier later.
        If petroleum jelly is thick enough to degrade the finish or detail on the surface you wish to cast, then thin the petroleum jelly with VM&P naphtha. You can find VM&P naphtha in hardware and paint stores. Do this in a well ventilated area and paint it on. This yields a much thinner film that will not reveal brush strokes or mute the details on the surface of your castings. Handle the naphtha with extreme care. It is a highly flammable solvent and should not be inhaled.
        Petroleum jelly is a mixture of mineral oil and “fumed”, “expanded” silica. This special industrial form of silica is a ‘thixotropic’ agent. It makes the mineral oil thick and sticky, except for the moment when you apply shearing action to it. When you scoop petroleum jelly with your finger, or brush it on with a paint brush, you are applying shear energy. This makes the jelly thin for a moment. After the shearing action stops, the jelly stays where you have put it. Expanded silica is the same stuff that keeps ink in your ball point pen from running into your shirt pocket. The ball shears the ink.
        If you have mixed VM&P naphtha with the petroleum jelly, the naphtha will simply evaporate very quickly. Naphtha will not dissolve the silica, and a thin film of oil and silica is left on your mold. Since oil and water don’t mix, this slimy, self-lubricating barrier between the mold and your cement or gypsum easily releases your finished casting from your mold.
        Strange but true, you will find that the grease or petroleum jelly seems to disappear into your casting. Not to worry, you can still paint your castings with water-based paints without paint adhesion problems. There are soft, porous, inert fillers in your cement or gypsum powder that soak up much of the oil. Cement contains unbinded clay that will readily absorb oil. Gypsum products contain fillers such as talc that will readily absorb oil. If adhesion of some particular brand of water-based paint is a problem, you can make your mold out of silicone rubber, which does not require any release agent.
        If you’re making full size bricks with cement, use grease or straight petroleum jelly. Cement tends to absorb more grease or petroleum jelly than gypsum does, because it takes longer to set.
        If you use grease or petroleum jelly as a release agent, suction and undercuts will be your only potential obstacles in removing castings from your mold. If you find suction is a problem, you can make molds that come apart. Or you can make your molds out of flexible materials, such as rubber.
        Undercuts are the nooks and crannies in your mold that grab and hold your casting when you try to pull it out of the mold. Sometimes you can solve an undercut problem by making a mold that comes apart. Sometimes you can modify the design of your castings to eliminate unintented or unnecessary undercuts. But, more often than not you will have to make your molds out of a flexible material to solve an undercut problem.
        The wooden brick mold in this project can be taken apart by just unscrewing it. If your mold is shaped as shown in figure 2 or 3, you will not have difficulty removing the finished brick casting from the mold.

figure 2			figure 3

        You will have a problem removing brick castings from a mold like the one in figure 4. The mold in figure 4 would have to be unscrewed each time you use it.


figure 4

        If you are an accomplished woodworker you can probably make a perfectly squared mold for your bricks. But don’t worry if you aren’t. The mold can be easily unscrewed to release your successful brick casting.
        If you are using an ice cube tray as your mold, you will want to use grease or petroleum jelly to release your casting. An ice cube tray is a mold for casting ice. The edges of an ice cube are rounded quite a bit. And your ice cube mold has some ‘draft’. This is the slight angle inward toward the bottom of each ice cube pocket in the tray. See figure 5.


figure 5

        Even a very slight draft angle makes release of your castings much easier. The only obstacle you will encounter releasing castings from an ice cube tray mold is suction. You will have to pry the finished casting out with a knife. Or cast a small loop of wire into the middle of your ice cube brick to use as a handle to pull the casting out. See figure 6.


figure 6

        Add the wire loop as the casting begins to stiffen, so it doesn’t sink into the mix.

        On the other hand, you may have found an ice cube tray that is made with particularly thin plastic. If the plastic is thin enough, you may be able to simply flex the mold and push on the bottom with your thumb, popping the casting out.
        If you’re making tiny bricks, to construct a model for indoor use, you may want to use gypsum. The bricks will harden quickly and won’t need to be waterproof. Bricks that won’t be handled much, say for use in scenery on a model railroad, can be cast with ordinary plaster of Paris from the hardware store. You can mix red water-based paint, food coloring, or fabric dye right in with the plaster to color it. The color effect will be realistic, and this process is far less tedious than painting each individual brick. Red “concrete pigment” can be used, also. Concrete pigments are available at construction supply stores, in powdered and liquid form. They are very strong, dense pigments. By volume, they cost about the same price as household paints, but you’ll get more pigment for your dollar.
        Use a plastic mixing bowl for the powdered plaster, and a paper cup for the water. Stir the mixture with a pencil or stick. Pour water over the powdered plaster slowly, mixing as you pour, until you have a creamy consistency, like pea soup. Don’t add any more water than you have to, though. The idea here is simply to make the plaster just workable enough to do the job. A runny, watery plaster will yield a weaker, softer, and more brittle finished casting. You want those molecules of calcium compound to stay as tightly packed as possible, not floating around in lots of water. And excess water will also slow down the setting time of the plaster.
        An unpacked, level, measured 8 fluid ounce cup of plaster weighs about 8 ounces. Companies that make plaster, like US Gypsum, suggest a mixture of about 1 quart of water to 2 pounds 12 ounces of plaster powder. So, for every level cupful of plaster you use, plan on adding about 7 fluid ounces of water.
        You may find that it takes making a couple of castings before you have a good sense for just how much water to add for a given project. It’s a good idea to make a habit of noting the quantities and proportions of ingredients in your mixtures. These notes will prove useful in planning larger projects, experiments, and mass productions. In mass production, measuring quantities for each casting will keep your material costs low, minimize waste (and cleaning), and insure uniform hardness and color in your finished castings.
        Although it may not seem that mixing is much of a process, how you mix gypsum and water, and how long you mix them have an effect. As you stir water into the powdered plaster of Paris, try as best you can to avoid entrapping air. Use a slow, consistent, circular stirring motion. When you have finished mixing, tap the bowl a few times on a table top to release bubbles. Your finished casting will be stronger, without weak spots due to air pockets. Your casting will also be stronger and harder if you mix it for a good long time. The longer the time you can spend stirring the plaster before pouring it, the better. Extended mixing time makes the mixture as homogenous as possible, making the strength of the finished casting uniform throughout, without overly dry and overly wet spots.
        When you’re finished mixing, pour the mixture into your mold, on a level surface. You can over fill the mold slightly and ‘screed’ the top by dragging a ruler or butter knife across it. Or you can wait and sandpaper the finished casting to smooth the top. Wait at least a half an hour before you take your casting out of the mold.
        Now, if you’re making bricks for an indoor dollhouse there may be some handling of the bricks in the use of the dollhouse. You can still use ordinary plaster of Paris to make these bricks, if this is an indoor dollhouse. But to make the bricks harder and stronger, add some wood glue (aliphatic resin glue) to the mixture. You can find wood glue at the hardware store, too. Mix the glue in equal parts with water and then pour it onto the powdered plaster. If you’re planning to use about 1 quart of liquid, mix up to 1 pint of glue to 1 pint of water. Don’t mix the glue directly with the powdered plaster. The dry powder will quickly pull the water out of the glue and form little, hard balls. Bricks made with wood glue take a little bit longer to set, but they will be much more durable. If you want the bricks to set faster, throw in a small amount of table salt. Or just use hot water in the mix to speed things up.
        An excellent, adhesive mortar for these tiny bricks can be made the same way. Mix wood glue with your water and add a touch of black paint, instead of red. For a look of very old mortar, add a touch of yellow or ochre as well. And to increase the working time of your mortar add just a little vinegar or baking flour. Either will slow the setting time of the plaster quite a bit. Using ice water in the mix will slow setting, too.
        Another useful additive for a plaster mortar would be either window cleaner or household ammonia. They reduce surface tension. They will help the mortar seep into the tiny pours in your bricks. You can add as much window cleaner or ammonia as you want. But don’t try to substitute isopropyl alcohol for window cleaner. Alcohols and sugars tend to stop plaster from setting.
        If your bricks have been sitting around for even a few days they will be dry. The water in your mortar will be quickly absorbed by the bricks. The mortar will stiffen on your brick, reducing your working time with the mortar. To prevent this, wet your bricks in a bucket of water first. Or, better yet, a bucket of water with some ammonia or window cleaner added. Use a wet cloth or sponge to clean excess and slopped mortar off the face of your bricks as you build. For added strength, a piece of thin, bare wire can be laid across each new layer of bricks, in the mortar.
        You’ll want to clean up the spilled plaster and plaster chips at some point. Use a very wet rag to wipe up any plaster that is still wet, and throw the rag in the trash. Sweep the chips into a bag. Or, better yet, sweep the chips into an old pillow case. You can tie the end, put the bag under your car tire, run it over, and use the crushed chips as filler in your next brick casting. (If you do recycle crushed chips, make sure to wet them in a bucket of water before adding them to your mix). Another cleanup idea that’s very convenient is to put all your equipment and material on a plastic sheet. When you’re done, just slide the hardened plaster lumps and chips into the pillow case. The plaster will not stick to a polyethylene sheet. Never put gypsum, or cement, down your sink or toilet. They’ll form a very hard blockage.
        If you need really strong, weatherproof, inexpensive bricks, you’ll want to make them out of cement. Working with cement is a little bit different from plaster of Paris and its gypsum relatives. Having to wait for the cement to harden may be your most difficult obstacle. But these rugged bricks may be worth the wait to you.
        By itself, Portland cement can be almost as brittle as plaster of Paris. As was the case with the plaster bricks, use only enough water to get the job done. The feel of cement is less granular than the feel of gypsum materials, because it contains a lot of clay. And with the smoother feel, you’ll probably find that you can get away with using less water than you would in similar casting projects with gypsum.
        Use “Portland cement”. Portland cement is a particular type of “hydraulic cement”, with carefully controlled characteristics. Although there are “quick setting hydraulic cement” products on the shelf, buy only straight Portland cement. Results with other cements will almost always be disappointing and unpredictable. Also, avoid the temptation or sales clerk’s suggestion to buy a mix, such as “concrete mix”. You don’t have a way of knowing what’s really in it. You will want to add your own “aggregates” for predictable results, anyway. And in the long run, you’ll find you have saved money and disappointments by buying and mixing your own ingredients.
        To make cement castings less brittle, you need to mix aggregates with the cement. You can cast in straight cement. But you can also catch the details of your design with a mix that contains a careful selection of aggregates.
        You can cast detail as fine as a fingerprint with cement or gypsum. But never mix gypsum and cement together. Such a mixture will indeed set faster than cement, but the finished casting will be soft. It also won’t be waterproof. The bond will be a plaster bond. Sulphur interferes with the chemical reaction in cement hardening. The plaster bond will not be a stong ‘glue’. And your aggregate will be the soft, amorphous clay in the cement powder. This mix might pass for a sidewalk chalk. But a better, more controllable, chalk mix would be plaster and talc. (Avoid talc powder that contains cornstarch; sugars and alcohols tend to stop cement from hardening, just like gypsum).
        Unlike gypsum, Portland cement is a very effective glue, because it cures almost as hard as a rock. Each grain of sand, pebble, chip, or whatever you add as an aggregate, is coated with hard cement, and locked firmly in place. A hard, cohesive mass is formed, just like the interlocking molecules of calcium compound in gypsum, but in a size you can see with your eyes. Individual pieces of aggregate will interlock best if their shapes are jagged. Sand and pebbles from the beach are not a good choice for an aggregate, because of their rounded shapes.
        Straight cement or gypsum yield a homogeneous, monolithic solid. That’s why they’re brittle. By adding aggregates you create a heterogeneous mass. Each individual piece of aggregate is small and unlikely to fracture. And, using aggregates, you don’t get the sort of molecular chain reaction from a shock wave that results in a fracture. When you drop your finished, fully cured casting on the floor, it is less likely to break. The shock wave is distorted and dispersed as it is transmitted from one type of material (your aggregate) to another (the cement). Going in many directions instead of one, the shock wave is weakened considerably. It turns into lots of little, scattering wavelets that do less damage to your casting (but maybe not the floor).
        To make your first cement brick casting you can take the same approach as with gypsum and just mix cement, water, and red coloring. Or, try adding an aggregate.
        You can add more than 1 part aggregate to 1 part cement powder. And a good place to start would be 3 cups of aggregate per 1 cup of powdered cement. Add some water to the aggregate, just enough to make it damp. Then thoroughly mix the aggregate and cement together. Having the aggregate damp does two things. The water will bubble out air pockets in the dry aggregate, so that you don’t end up with lots of entrapped air in your casting. And, the cement powder will completely cover, and stick to, the surfaces of each piece of damp aggregate, for a good gluing effect.
        If you’re using a powdered concrete pigment, add it to the damp or dry mix also. If you’re using red paint for color, just pour some over the top of the damp mix. Next, add water slowly, as with gypsum, stirring and pouring just until you have a usable consistency. There’s no need to hurry mixing cement. It takes a while to set.
        As with gypsum, you can use hot water to coax the cement to set faster. It also helps to keep your casting in a warm place. Avoid letting your castings set at below fifty degrees. A powdered cement accelerator, calcium chloride, is sold in stores that sell cement. It works well, and you don’t need much. Add only up to about 2 percent by weight. To 1 pound (16 ounces) of cement powder add about a third of an ounce of calcium chloride (16 ounces x 0.02 = 0.32 ounces). Add the calcium chloride powder to the damp mix.
        By volume, an unpacked, level, measured 8 fluid ounce cup of Portland cement weighs about 6 ounces. And a measured cup of calcium chloride weighs about 8 ounces. So, for every measured cupful of Portland cement, you can add about 3/4 of a measured teaspoon of calcium chloride. (6 fluid ounces x 0.02 = 0.12 fluid ounces). Doing the math for conversion, this is about 1/8 of a fluid ounce, which is about 3/4 of a measured teaspoon. Precision here is not critical.
        High early Portland cement is much more expensive than ordinary Portland cement, but it can set almost as fast as gypsum. You can add aggregates to high early cement as well. One brand of high early Portland cement that works well is EUCO K-MENT , made by The Euclid Chemical Company, 19218 Redwood Road, Cleveland, OH 44110. Don’t add calcium chloride to this product, though; it crazes surfaces on your castings.
        A less expensive, fast-setting alternative to high early cement, for tiny bricks, would be ordinary “mortar mix”. It sets fast, but it will crack if you try to use it to make full size bricks. It’s intended for thin applications, up to about half an inch thick. You’ll need lots of red coloring, and you can add calcium chloride to mortar mix.
        Using straight cement, you can just pour your mix into the mold, as with gypsum. And you can add ammonia or window cleaner to reduce surface tension on the mold surface. If you use lots of aggregate, you’ll want to pack the mixture into your mold. Holding a spoon under your thumb and pressing does a good job.
        If you’re using ordinary Portland cement, let the casting set for a week before removing it from your mold. Cover your mold with plastic wrap. Then store the brick in a plastic bag for up to a month, to keep it damp. This will give the cement a chance to harden. The important thing here is to keep the brick from drying out. Cement that stays damp for a month will have maximum strength. Working with cement requires lots of patience; working with gypsum does not. If you work with cement, the trick is to have many molds.
        If you’re in a hurry to cast bricks, use cement accelerator or high early cement. If you use cement accelerator, hot water, and about 90 degrees room temperature, you’ll be able to use your bricks much sooner. Bricks made with high early cement can be ready to handle in 24 hours.
        Wood glue can also be added to your cement mix. This will increase your material cost, but your bricks will have early ‘green strength’. You’ll be able to take them out of the mold sooner. Wood glue will also make your cement casting less brittle, and the glue is water resistant. As with gypsum, add the wood glue to your water, not directly to your damp or dry mix.
        Commercial “mortar mix” or “mason mix” can be used as mortar to build with cement bricks. Or, you can make your own mortar by mixing Portland cement with “lime” or “whiting”. Both lime and whiting are crushed calcium carbonate, a sort of fine, gritty (‘sharp’, jagged), white sand. Whiting is crushed more finely than lime, and both are excellent aggregates. But because of their bright, opaque, white color, they are not a good aggregate choice for your bricks. Your bricks would turn out pink instead of red. A better choice for a brick aggregate is a golden colored sand.
        Wood glue would add stickiness to your mortar. And you will want to wet your bricks in a bucket of water before building if your mortar stiffens on the brick.
        Another useful additive for cement mortar is a small amount of dish detergent. Dish detergent reduces surface tension, just like ammonia and window cleaner, but it also makes the mortar feel smoother to your hands. You’ll avoid the abrasive feel on your fingers caused by the calcium carbonate in mortar mixes. Avoid adding dish detergent to your brick mix, though. The detergent will leave surface bubbles on your casting, and it will cut through the grease or petroleum jelly on your mold surface. Although dish detergent reduces surface tension on a hard surface, the detergent creates bubbles on top of itself. While this type of bubbling would be a problem in a casting, your mortar surface is exposed. Detergent bubbles form on top of the exposed mortar, not trapped against the wall of a mold.
        Another type of brick you may want to try casting is refractory fire brick. Maybe you’ve considered the idea of building a backyard fireplace, a kiln, or a furnace to melt metals. Bricks are a convenient way to build these. You can use cement/sand bricks to construct a rugged external structure. And use fire bricks to line the inside, to handle flames and high temperatures.
        Cement and red clay bricks would pop, crack, and even explode at high temperatures. High temperatures, anything above 211 degrees, will boil the water right out of your bricks. Cement bricks will begin to crumble as they are being calcined. But fire bricks will stay together and contain the heat.   To make fire bricks …





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Copyright © 1997, 98, 99, 2000 by Wayne Pearson.
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