This instructable will narrate the process of creating a ceramic sculpture from a 3-d scan, which includes mold making for slip casting ceramics and hand carving a plaster model sconce for display.
Step 1: Tools and Materials
Suggested Materials for mold making and carving plaster
Soft Clay – Any type of smooth clay. The cheaper the better as much will become waist. This will be used for making models and building partitions to control fluid plaster.
No1. Pottery Plaster- The best for slip casting, available at most ceramic supply stores.
Oil Soap- The brand name stuff used on wood floors and furniture is found at the grocery and hardware store. Pottery Tools- Metal rib, large loop trimming tool, round sponge. Found at most art supply shops.
Razor Knife- Useful for carving plasters hard to reach places, as well as finishing cast parts
Rasp- small cheese grater style with handle. This is an important plaster carving and finishing tool, available at the hardware store.
Buckets- Two 2.5 or 5 gallon plastic buckets for mixing water and plaster.
Pound Scale- This is for weighing plaster and water-find one that weighs 10 -25 lbs.
Electric Drill and Paint Mixing Blade .- used for mixing plaster.
Cottle Boards- These will need to be made from quality plywood and 2x2 pine stock.
Clamps- C- Clamps or ratcheting style, I prefer the ones with the grip. These are used for holding cottle boards together.
Rubber Mallet- From the hardware store. Sometime plaster pieces don’t come apart so easily.
4 Inch Putty Knife- Used to scrape and clean up plaster residue.
Brushes- Cheap ones from the hardware, one inch wide is fine, used to apply the soap to plaster and work surfaces.
Sand Paper- Get a couple different types, Drywall screen and 400 mesh wet sandable.
Chisels for shaping plaster.
Vinegar- Used to clean up cast parts and also to remove and oil soap from the mold interior.
Straps or rubber bands to hold the mold together while casting.
Casting Slip- Commercially Available, or many formulas can be found online.
Step 2: 3-d Print
A marble bust of Alphonso Taft was scanned using structured light. The model was scaled, exported as a .stl file, powder printed, and sealed with cyanoacrylate.
Step 3: Parting Lines
The first step in making a mold of an object that has multiple sides is in determining the parting lines on the shape to be molded from. Parting lines are the boundaries of the mold parts, and in this we must first get farther into the concept of undercuts.
If you take any object with varying degrees of curvature and look at from a fixed vantage point/ one angle, you cannot see the whole form, there are sides and parts that are hidden from view.
Looking here is an effective way of dividing a multiple sided object in to parts that can be successfully molded and used for slip casting. Hold up the object and turn it around looking carefully at its shape. Without moving your head, only the form, find a side where there is the most surface area in view, this is most likely the first piece of your mold. Any surface that is hidden or tucked away around a corner cannot be taken as a mold piece. With a marker, and without changing your perspective towards the form, draw out with dots all along the border of the area you can see. Look very closely and place the dots visually in half along the edges of your boarder. This is the first mold part. Next rotate the form to the left until the dots from the border of no.1 and are almost lost and stop. That line of dots becomes one side of the parting line. For side no.2, fix your vantage point and investigate the next visible surfaces. Draw out the perimeter of the entire visible surface are as before. Continue these steps until there are no unresolved surfaces left on the form.
Next lay your model on a smooth clean surface and draw a square around it with an inch and a half of space from all sides of your centered object. This line represents where the cottle boards will be placed later.
Step 4: Clay Wall Division
After applying a layer of soap and water solution to resist the plaster, build wall of clay all around the model hiding everything but the selected surface area for a mold piece. Bring the clay all the way to the dotted line, splitting it in half. You do not want the line hidden nor do not want to see all of it for the cleanest most precise mold that will release easily.
When building up the clay, think of the vertical walls of the cottle boards and make the clay bed as perpendicular as possible forming a right angle. Work on most of the clay without the boards up, as it is easier to get your hands around the model. Make the clay as smooth as possible. This will limit the amount of carving and sanding that will need to be done on the cast mold part
Once most of the clay has been built up line up the boards as previously drawn out on work surface and clamp them in to place. Make sure that there is 1.5 -2 inches of space between the model and the boards as this is the thickness you want the plaster. Finish up the clay on the interior flush to the wall with no visible gaps. Use the rubber modeling tool if necessary to compress the clay against the sides. Roll up coils and put the bead on the outside of the boards to seal off any opportunity for plaster to escape. It is now time to pour the first mold part.
Step 5: Working Around the Part, Key Registration and Carving
No 1 Pottery Plaster works best for the absorption of water from the casting slip.
The Proper ratio of plaster to water is 1:.07. For instance ten pounds of plaster requires seven pounds of water. The Plaster and water should be weighed in separate buckets. Always add the dry plaster to the water. For very small amounts the plaster can be mixed by hand.
When making a piece mold it is necessary to make sure the plaster has been weighed and mixed precisely and consistently. Follow the steps and preferably mix mechanically with a drill and paint mixing blade. When mixing mechanically the blade should be introduced to the plaster mix at an angle an inch from the side of the bucket. If necessary tilt the bucket to prevent whipping too much air into the mix.
Slake ( carefully adding plaster to water) over the course of a minute. Cool water should be used as the warmer it is the faster the plaster will set up. Agitate the mixture violently for just over a minute. Reduce the speed and mix slowly for another minute or so to get rid of air bubbles. Tap the side of the bucket to further the release of air and the mix should nearly be ready. Let it sit and check the consistency by running your finger across the surface of the mix, once a visible line or wake remains it is ready to pour.
The purpose of precision and accuracy of weighing and mixing plaster and water is that each mold piece needs to be of the same density and composition to draw out the water from the clay evenly. For instance if one mold part is more porous and another more dense a section of the cast can become thin and uneven which can prevent the cast from proper release, cracking, and other unpleasant issues that are typical of different thicknesses of clay.
Each mold part that is cast needs some treatment to prepare for its neighboring part.
The first piece has been poured, set up and cooled. Take the boards apart and unearth both the model and the plaster piece. Sides that were cast against clay need to be planed true and square. A loop trimming tool works great for shaving plaster as well as a metal rasp. Be careful to not carve into the detail of the inside of the mold piece. Shave it, cut in the keys and sand the side that that will be touching plaster next with 400 grit wet sand paper. Where a mold piece meets another consideration must be given to register the parts together to help with the proper directional removal of plaster pieces. These keys need to be carved out of each previous part to help lock multiple pieces together.
The half round coarse file can make nice dart like keys for parts that need to slide away from each other in specific directions.Make it as smooth as possible
Plaster by nature wants to stick to itself so a release agent is needed before plaster can be poured onto plaster. This process seals up the surface but can be removed later. Commercial mold soap is available but oil soap can work as well. I use a 50% solution of water and oil soap as a parting compound. Brush the soap onto the first mold piece, rub it in and let it dry. Cover all areas of the plaster including the outside that will be against the cottle boards, just incase there is any plaster spillage. The plaster should be soaped a minimum of two times or more, allowing the surface to dry before continuing.
With each plaster part poured there are fewer clay walls that need to be built. Much like the activity of mapping out the parting lines, you will pour the mold parts sequentially. Repeat the steps for each piece until the entire model is encased with plaster.
Remember where a mold piece meets another, consideration must be given to register the parts together to help with the proper directional removal of plaster pieces. These keys need to be carved out of each previous part to help lock multiple pieces together.
Step 6: The Details
Detail of keys for directional mold part removal and clay wall division of parting lines. Often plaster will have to be poured to the highest elevation of the parting line. Then the plaster will be carved down to follow the line and make space for the next piece.
Step 7: Finishing Up the Mold and Recap of Procedures
The finished multiple part mold will go through a couple stages of cleansing. First off, all soap needs to be removed from the casting surface. With the sponge and a 50/50 water vinegar solution scrub off all soap from the interior of the mold. Once the mold has been washed, dry it out, and hole will have to be carved into bottom mold part as pouring gate for the clay slip. The first few castings will also help with the cleaning of the mold, as the slip will grab some excess soap off the plaster.There is a good chance that the first several castings may fail as the mold becomes accustomed to its new job. Of course start the day off with a clean dry mold.
Step 8: Casting and Firing
Once the mold is clean and dry strap it together and pour the casting slip. The material will remain in the mold until the desired thickness is obtained. When ready the excess slip is poured out and the mold will sit until the clay shell becomes rigid enough to hold form. Mold parts are then sequentially removed revealing the clay reproduction. Clay shrinks so think of that before hand. The digital model for this project was scaled considering the shrinkage of the porcelain to obtain the desired dimensions.
Step 9: The Sconce
Beginning with a block of plaster and drawings to scale I traced the basic profiles of the sconce onto all sides of the plaster. This gives is good sense of the shape to begin roughing out with hammer and chisel, and is redrawn periodically as the form took shape. Once the roughing is done the loop pottery trimming tool is used to shave the sweeping form. A combination of chiseling, rasping and shaving were used throughout the process to reduce the block into decorative detailing.
Step 10: Wet Sanding and Fit
Final fit and finish was completed by sanding with drywall screen to remove the harsh marks of the tools and was wet set sanded with 400 mesh sandpaper to a near polish. Finally a mold was made from the plaster sconce in the same fashion as the bust.
Step 11: Finishing Touch
After the high temperature glazing, the surfaces were hand painted and fired multiple times with overglaze enamels and gold luster.