Making a mold,Pouring
& Slip Casting
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Threading Instructions for cloth mold straps:
The white side of strap in the diagram is the side that faces away from the mold.
The gray side of the strap in the diagram is the side that is against mold.
Follow diagram & snap buckle in place.
Compared to the metal buckle, mold plaster is rather soft.
Insert a small sheet of corrugated cardboard under the metal buckle to protect the mold from damage.
How to... Make a plaster mold.
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| Materials Necessary: |
Tools Necessary: |
- Model
- Modeling Clay
Water base such as WED or #10.
- Plaster
USG No.1 Pottery
- Orange Shellac
For sealing clay and other porous objects.
- Parting Agent
Tincture of Mold Soap or Purelube.
- Baby Powder or Talc
For dusting prior to applying parting agent.
- Denatured Alcohol
For cleaning shellac brushes and thinning down shellac.
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- Wood/Wire Tool
- Wood Sculpturing Tool
- Metal Spatula or Knife
- Casting Boards (4)
- Work Board
- "C" Clamps (4)
- 3" Putty Knife
- Screwdriver
- Hammer
- Rubber Mallet
- Round File (coarse)
- Ruler or Yardstick
- Metal Spatula or Knife
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- Assorted Brushes
- Scale (0-60 lbs.)
- Metal Square
- Wooden Wedges
- Drill Motor, 1750-2400 rpm
(shaft with 3" rubber disc) or Large Enamel Spoon
- Scooper
- 5 Gallon Plastic Container
- 1 Gallon Plastic Jar
- Stainless Steel Scraper
- Rubber Bands (banding)
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Procedure:
Models made of clay, wood or plaster must be coated with 2 or 3 layers of thinned down orange shellac. |
Step 1: First you must study the object from which you are to make the mold, to establish the Parting Line. Draw a line on the object with a dye-marking felt tip pen. |
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Step 2: Place the object on your Work Board so your parting line is somewhat parallel to your work surface. Proceed to Clay-Up with water base modeling clay around the object to your parting line, extend out for one inch from the widest points. |
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Step 3: When clay is all in place, smooth and leather hard, square off clay as shown. Using a soft brush, apply two thin coats of Orange Shellac over the object and the top surface of the clay parting line. After 15-20 minutes, apply talcum powder, dusting lightly. |
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Step 4: Apply Parting Agent with a soft brush covering the entire surface. Dry your brush and pick up all excess Parting Agent, leaving a very slick surface on the object and parting line clay. |
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Step 5: Prepare your casting boards, wiping each with Parting Agent on front surface and edges. Assemble as shown with "C" Clamps. With wooden tool, seal edges where clay parting line meets the insides of the board as shown. |
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Step 6: READY FOR CASTING. For the size of the object shown in these drawings and the apparent bulk of parting line around the model, we will use the following proportions: With your 1 gallon plastic jar, weigh out 1 1/2 lbs. of No.1 Pottery Plaster |
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Step 7: Let the plaster soak for 3-5 minutes, then mix with your Drill Motor Mixer. Mix for about 1 1/2 minutes, then pour over boxed pattern. Plaster should cover at least 1 inch over the highest point on the Model. |
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Step 8: After 20 minutes, take casting boards apart, scrape off top of plaster and bevel the edges slightly. Grasp opposite sides of the plaster/clay mold and gently twist to loosen clay from the work board as shown. Turn mold over and proceed to lift off clay from model and plaster half of the mold. |
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Step 9: If orange shellac transfers to plaster half, clean with alcohol. Clean all clay particles from the mold surface. Now is the time to carve Keys into the plaster parting line. You can make Keys several ways - round end of large spatula, a coin (nickel or quarter). |
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Step 10: Prepare your casting boards again, dust the model and parting line area with talcum, brush on parting agent as in Step 4. Assemble the boards as in Step 5 with "C" Clamps and repeat Steps 6 and 7 for casting the second half of the mold. Let the plaster set for 1/2 hour, remove the casting boards, scrape top of the mold, bevel the edge and corners. |
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Step 11: Now you are ready to open the mold. Scrape off any plaster that may have run down the side of the first half of the mold. Using a flat end screwdriver or a wooden wedge, insert it at the parting line; tap it gently with the hammer. As soon as the mold starts to part, turn the mold over and repeat the process. When the mold is loose, grasp each half and gently pry apart. |
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Step 12: The model will usually stay in half of the mold. At this point, how accurate you were with your parting line, what material your model was made of , and how hard or soft that material was, will dictate how easy or hard it will be to get the model out of the half plaster mold. |
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Step 13: If the model was made of clay, you can ease it out by using the screwdriver. But if you have to do any prying like a lever, place a flat piece of wood under your screwdriver so you won't chip the plaster mold. If your model is made of metal, glass or ceramic, you might have to use other means, such as air pressure or tapping all around the model with a rubber mallet. In some cases, I have chipped out the model thus destroying it. But keep in mind, that at this point the mold is the main object because a good, usable mold can reproduce 50 to 150 objects. |
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Step 14: With both halves now clean of any particles, we now determine where to carve in your Pour Hole. It can be in one half or in both halves as shown. Bevel the outer edge of the parting line on both halves and bevel all outside edges of the mold. This keeps that edge from chipping. |
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Step 15: At this time, check each half of the mold for any under-cuts that can be cut back. Let your mold dry out for 4 or 5 days depending on your weather conditions. NOW YOU HAVE A MOLD. |
Glossary of Terms:
- Clay-Up - Filling in around a model to develop the parting line.
- Leather hard - Clay dried to consistency of leather.
- Model - Object that is used to make a mold over.
- Parting Agent - A material used over an object that leaves a slick film.
- Parting Line - The line which divides your model.
- Work Board - Board on which the model is prepared for casting.
- Dry Brush - Wiping your brush so it will pick up excess material from the model or mold.
- Keys - Round or oval shape depressions with matching counterpart usually on flat section of parting line of mold.
- Pour Hole - Carved inlet or opening in mold to pour ceramic or porcelain slip into.
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Making your own Casting Boards
Casting Boards are used to form a box around a clay-up object before casting your plaster mix into it.
These drawings show two different ways to make corner angles for clamping the boards together.
Boards can be moved in or out from each other to make a small or large square or rectangle. Boards of varying sizes can be made. |
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| How to... Make and adjust casting slip. |
Making good slip is the first and most vital step in a successful slip casting operation. Although there are those who will say they have been using the same Dixie Cup for the last ten years to measure Sodium Silicate, and a nail on a wooden barrel for a water marker, this approach leaves far too much room for error.
Accurate measurements of Viscosity and Specific Gravity are important. It doesn’t take a ceramic engineer to make a good quality casting slip, but slip mixing is a chemical process that is affected by even the slightest changes in temperature, humidity and the chlorine and chemicals that are added to water supplies. Changes in conditions such as weather and water can sometimes affect the mixing formula, but if you adhere to the following procedures, you will consistently make good slip.
It is common to experience significant variations in the amount of deflocculant required from one clay body to another. Low-fire, talc bodies, for instance, require considerably more deflocculant than Cone 5 porcelain bodies. It should be noted that our suggestions are based upon average climatic and water conditions in our laboratory in Southern California. Adjustments may be necessary to fit your climate, water and general studio or plant conditions. Also, for best results, make your slip 24 hours before using it for casting. |
| Equipment & Chemicals Necessary for Slipmaking |
- Slip Mixer
With a motor and blades which are capable of properly mixing your batch of slip.
- Gram Scale
With 500g attachment weight. A postage scale is not accurate enough.
- Viscometer
The principal quality control tool for slip. It is used to determine Specific Gravity, the ratio of clay to water in the slip, and to measure Viscosity, the property of flow.
- Liquid Ounce Measuring Cup
Sodium Silicate, for instance, must be measured by the liquid ounce, not by weighing it on a scale. 4 ounces of Sodium Silicate, measured by volume, weighs approximately 5 ounces on a scale.
- Respirator Mask
NIOSH approved.
- Sieve or a Shaker Screen
A Talisman Glaze Sieve (TM-363 in this catalog) is an invaluable aid in screening your slip. You simply turn the handle and the slip is forced through the screen. 60 mesh or finer recommended.
- Clock or Stopwatch
To measure seconds for the Viscosity test.
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- Sodium Silicate (N Brand)
Sodium Silicate is a chemical used to deflocculate casting slip. Too much Sodium Silicate causes overdeflocculation. (See the section on Over-Deflocculation.) Sodium Silicate may be stored for extended periods in tightly closed plastic containers. (Aluminum, galvanized iron or zinc containers SHOULD NOT be used.)
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NEVER ADD UNDILUTED SODIUM SILICATE TO THE SLIP BATCH. |
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NEVER ADD ENTIRE PORTION OF SODIUM SILICATE TO THE BATCH ALL AT ONE TIME. ADD ONLY IN SMALL AMOUNTS. |
- Soda Ash
Soda Ash works to dissolve lignite in clay. It works in combination with Sodium Silicate to aid in deflocculation. The correct combination of the two will give proper casting qualities, however, if only Soda Ash is used, your clay will become sticky. If Soda Ash is not stored in tightly closed containers, a chemical change occurs and it becomes Sodium Bicarbonate (Baking Soda). If this occurs it will then act as a flocculant (thickener) instead of as a deflocculant.
- Barium Carbonate (Toxic in raw form)
Barium Carbonate neutralizes (1) sulphates present in the clay or in the water and (2) sulphates leeched from molds used in casting.
- Liquid Ounce Measuring Cup
Sodium Silicate, for instance, must be measured by the liquid ounce, not by weighing it on a scale. 4 ounces of Sodium Silicate, measured by volume, weighs approximately 5 ounces on a scale.
- Dispersal (Darvan)
Dispersal is an organic deflocculant that can be used with less fear of over-deflocculation. It is especially effective in hard water areas. Darvan 7 is recommended for most applications, Darvan 811 is best for red slips.
DO NOT ALLOW DISPERSAL TO FREEZE.
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Mixing Formulas & Procedures |
You will find that different proportions of materials and chemicals will alter the performance of your slip, making it more or less desirable in your process. Therefore, we do not provide precise amounts of the ingredients in our formulas. Instead, a starting minimum amount is given. We recommend that you begin with these amounts and, if adjustments are needed, slowly and carefully add additional quantities.
A formula using 100 lbs. of dry clay blend will yield 10 gallons of slip. Remember, start with the minimum amounts shown below.
- Clay Blend
100 lbs.
- Barium Carbonate
1/2 ounce (14.75 gr.)
- Soda Ash
1 ounce (23.35 grams)
- Water
5 gallons (19.50 liters)
- Sodium Silicate
2 1/2 fluid ounces. You MUST dilute this amount with a like amount of water. This will yield 5 ounces of diluted Sodium Silicate.
SODIUM SILICATE SHOULD ALWAYS BE DILUTED 50/50 WITH WATER BEFORE BEING ADDED TO THE BATCH.
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- Before you begin mixing, put on a good quality respirator to protect you from inhaling dust.
- Measure water carefully and pour into the slip tank.
- Accurately measure the minimum amount of Sodium Silicate indicated for your batch and add a like amount of water (i.e. 5 oz. Sodium Silicate + 5 oz. water) in a measuring cup. Set aside. NEVER ADD SODIUM SILICATE FULL STRENGTH TO A BATCH OF SLIP.
- Accurately measure the amounts of Soda Ash and Barium Carbonate indicated for your size batch.
- Add Barium Carbonate and Soda Ash to one quart of warm water. (Warm water aids in mixing the chemicals.)
- Turn mixer on. Add diluted Barium Carbonate and Soda Ash (which you prepared in Step 5) to the water that is in the tank. Mix for 5 minutes.
- While the tank is still mixing, slowly add 1/3 of the clay indicated for your size batch to the water.
- Slowly add 1/3 of the Silicate solution (as mixed in Step 3) to the batch. NEVER ADD PREMEASURED SODIUM SILICATE TO THE BATCH ALL AT ONE TIME. ADD ONLY IN SMALL AMOUNTS.
- Mix approximately ten minutes, allowing the Sodium Silicate to thin the batch, then add 1/3 more of the clay. Allow clay to mix into the batch.
- Add 1/3 more of the Sodium Silicate solution. This will again thin the batch.
- Mix another 10 minutes, then add the final 1/3 of your clay to the mixer.
- Add the last 1/3 of the Sodium Silicate.
- Let the mixer run according to the following table, or until all lumps are blended away:
100 lbs. = 2 hours 1000 lbs. = 4 hours
300 lbs. = 3 hours 2000 lbs. = 5 hours
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Test Procedures
To determine if the slip is properly prepared, you need to measure two properties: Specific Gravity and Viscosity. You should measure and adjust Specific Gravity before you test and fine-tune the Viscosity. Specific Gravity can be tested after the mixing time indicated in the table above. Viscosity should be tested no sooner than 12-24 hours after the batch is mixed. This allows the clay particles time to hydrolyze, or become wet on all surfaces. If the viscosity is adjusted before hydrolyzation is complete, you are likely to over-deflocculate the slip. |
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Specific Gravity Test
After all lumps are blended away.
- Remove the stopper from your viscometer and weigh the dry, empty flask.
- Fill the flask with slip up to the 500ml mark.
- Weigh the filled flask on your gram scale.
- Subtract the weight of the flask from the total weight. The result will be the net weight of 500ml of slip.
- Divide the gram weight of the slip by 500 to establish the Specific Gravity. Example: A slip weight of 875 grams ÷ 500 gives a Specific Gravity of 1.75. Experience will teach you which weight is most appropriate for your casting conditions, and most casting slips work best between 1.72 and 1.80 Specific Gravity.
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Viscosity Test
12-24 hours after batch is mixed.
- Fill the flask to the 500ml mark and insert the stopper.
- Hold your finger over one hole in the stopper and turn the Viscometer upside down.
- Release your finger from the hole and time the flow of slip until you count one second between drops. Normally the slip will flow through in 100 to 130 seconds. A fast flow time (thin slip) can lead to a slower setting time and more brittle ware. A slow flow time (thick slip) will set up faster in your molds but render ware that is less dense.
- Tips:
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If the slip weight is normal, it is highly unusual for the flow rate to be less than 100 seconds per 500ml. |
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If the slip weight is normal and the flow time is more than 130 seconds, you may need a little more diluted Sodium Silicate or Dispersal. Never add water - it changes the Specific Gravity. |
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After testing, always wash your Viscometer with cold water and store at room temperature. |
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Adjusting the Batch
If your Specific Gravity is too low, add more clay to the batch. Then you need to wait while the addition mixes completely through the batch. With every clay addition, you should mix the batch for the fully recommended time (i.e. 3 hours for a 300 lb. batch).
If the Specific Gravity is too high, add water. If the Viscosity is too high, add deflocculant. You need to wait just twenty minutes for these additions to mix through the batch before re-testing.
If your Specific Gravity or Viscosity are not within the recommended limits, but your slip casts well, you should not make any adjustments! |
Over-deflocculation
Sodium silicate is the most common economical and powerful deflocculant. Unfortunately, it can quickly turn from your friend to your foe. Initial additions of sodium silicate serve to thin (deflocculate) the batch. However, there is a point after which sodium silicate starts to have the opposite effect, actually making the batch thicker (flocculating it). Compounding the difficulties is the fact that sodium silicate accelerates its effectiveness as it nears the point of over-deflocculation. For example, you might add one ounce to the batch three times, with little improvement in Viscosity, but the fourth addition will send the batch over the limit. That is why it is recommended to use small additions until you are familiar with slip making.
If you add sodium silicate and the slip gets thicker instead of thinner, it is over-deflocculated, and you have two options: you can try to recover the batch or throw it away. If your slip is badly over-deflocculated, it is very difficult to correct and will cost you more in time and frustration than the purchase of new slip. If the batch is slightly over deflocculated, you can bring it back with the addition of more dry material. Determining the correct amount will be trial and error, but with every clay addition, you should mix the batch for the fully recommended time (i.e. 3 hours for a 300 lb. batch).
If you believe your are close to over-deflocculation, it is time to switch from sodium silicate to dispersal. Dispersing agents such as Darvan can be added after you have added the maximum amounts of Sodium Silicate. You may add as much as 1 ounce of dispersal per 100 lbs. of dry clay without fear of over-deflocculation. |
How to Pour Ceramics Slip
Into a Plaster Mold
This is how to make greenware. The plaster mold absorbs moisture from the slip, leaving a clay shell.
Steps: 1. Make sure the mold is clean and dry. 2. Close the mold, fastening it shut with rubber bands or mold straps. 3. Pour the slip into a container that will hold more than enough slip to fill the mold. 4. Stir the slip with a wooden paint stirrer to be sure that it's mixed well. 5. Pour the slip steadily into the pour hole of the mold. The plaster will absorb moisture from the slip, forming a layer of clay. 6. Keep filling the mold until the drying clay forms a 3/16-inch wall. This is a general rule; the thickness will vary depending on the size of the mold. (Smaller pieces are typically thinner.) 7. When the piece is thick enough, pour the excess slip out of the mold and set the mold upside down over a container (such as a bowl) for about 10 minutes. 8. Turn the mold upright and give the clay time to harden ' 15 minutes or so should be sufficient. You want the piece to be able to stand up by itself without folding or bending when it's removed from the mold. 9. Trim the extra slip from around the pour hole. 10. Put the mold on its side on a flat surface. Gently lift the top half of the mold straight up. If it feels like it's sticking, it isn't dry enough yet ' let it sit another 15 minutes. 11. Let the piece sit for another 5 minutes after the top half of the mold is removed. 12. Turn the piece over in your hand so that the other half of the mold is on top. If the piece still feels soft in your hand, let it dry longer before trying to remove the second part of the mold. If it feels like it can support itself, remove the rest of the mold. 13. Put the greenware somewhere safe, warm and clean to dry overnight. Tips: Wipe out your mold before and after you use it. Securing your mold with the mold straps or rubber bands will prevent warping as the mold dries. Warnings: Don't ever jiggle a mold half to pull it loose. Let the piece dry longer if it sticks.
How to Clean Ceramics Greenware
This is something you'll want to do carefully. Once the piece is fired, there's no going back.
Steps: 1. Collect your tools. You'll need a scraping tool, a texture tool, a spoolie and a sponge. 2. Be sure that the greenware is completely dry. If it's still soft, you risk denting or poking a hole in it. 3. Scrape the mold seams with the straight edge of your cleaning tool. Go over the area lightly with fine sandpaper and then wipe it lightly with a slightly damp sponge to smooth the surfaces. 4. Use a texture tool to fix spots where the texture has been removed during cleaning. 5. Turn an item that needs to be even on top, such as a cup or bowl, upside down on a piece of fine screen or sandpaper placed on a flat surface. Twist it gently. Rubbing it back and forth will create unwanted grooves on the rim. 6. Use a spoolie to clean small holes. 7. Apply underglazes at this time if you plan to use them. Your project is now ready to be fired. Tips: The cleaning tool has two ends. The sharp, triangular end is best for cleaning curved edges and flat surfaces. The round end is best for grooves and areas with lots of detail. There are many different kinds of texture tools. Two good, all-purpose ones to have are a stylus and a tool that resembles a small wire brush.
How to Hide a Pour Opening in Greenware
A gaping hole in the bottom of your project can ruin the look of an object like a ball or an egg. Here's how to hide the pour opening.
Steps: 1. After taking the greenware from the mold, turn one half of the mold face down, so the flat, smooth side is up. 2. Pour a puddle of slip on that flat surface, just slightly larger than the hole you plan to fill. 3. Put the project on top of the puddle and press down gently. 4. Let it stand there for at least a half-hour. 5. Lift the object straight up. It's ready if the clay lifts with the project. If it sticks to the plaster, let it sit longer. 6. Wipe the edges of the plug with a damp sponge until you can't tell the difference between the original and the addition. Tips: Wipe down your mold before and after you use it. Secure your mold with the mold straps or rubber bands to prevent warping as the mold dries. Warnings: When the project is fired, there must be a hole to let the moisture out. If you don't put one in it, the item will explode in the kiln. A toothpick is ideal. Keep in mind, if you plan to use glazes on your piece, not to plug the hole with glaze.
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ACRYLIC PAINTS / BISQUE STAINS
