Plaster Mixing 101: How to Mix Plaster for Ceramic Molds. How to Mix Plaster by
Guy Michael Davis • February 3, 2020 • Read Comments (10)
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Learning how to mix plaster is essential if you are interested in slip casting, but even if you’re not, mixing plaster is a good skill to learn. Plaster has a lot of uses in the pottery studio from plaster drying bats, to simple hump molds. Getting the plaster right can be a little challenging if you are new to it so today we wanted to give you a primer on how to mix plaster. Hopefully, this will give you all the resources to help you better understand and use this essential pottery studio tool. In this video clip, an excerpt from his video, Fundamentals of Mold Making and Slipcasting, Guy Michael Davis shows us how to mix plaster and pour a one piece mold.
We’ve also included what we think are the 10 Steps for Success with Plaster as well as the handy dandy Plaster to Water Mixing Chart below. Having all of these resources in one place, should make your next plaster project a piece of cake. – Jennifer Poellot Harnetty, editor.
REMEMBER TO USE GOGGLES AND GLOVES
Ceramic Mold Making Techniques
How to Mix Plaster – 10 Steps
1. Prepare your mold.
A common mistake of potters is to mix plaster only to realize everything’s not set up for pouring. Before casting, make sure your model is set, the mold boards or cottle are secure, and all the surfaces you’re pouring onto are coated with a parting agent such as mold soap.
2. Prepare your work area.
You will need a clean mixing container for the plaster, a scale for weighing the plaster, a measuring cup for the water and a rinse bucket. Note: Plaster cannot be permitted to go down the drain, because it will form a rocklike mass. Even small amounts will accumulate over time. Line a rinse bucket with a plastic garbage bag and fill it with water for rinsing your hands and tools. Allow the plaster to settle for a day, then pour off the water and discard the bag.
3. Use fresh water.
The mixing water you use should be at room temperature or 70°F (21°C). If the water is too warm, the plaster will set too fast and vice versa. Use only clean, drinkable tap water or distilled water. Metallic salts, such as aluminum sulfate, can accelerate the setting time, and soluble salts can cause efflorescence on the mold surface.
4.Use fresh plaster.
Plaster is calcined, meaning chemically bound water has been driven off through heating. If the plaster has been sitting around in a damp environment, it will have lumps in it, in which case it is no longer usable. Pitch it. Use plaster that has been stored dry and is lump free.
5. Weigh out materials.
Do not guess about the amounts of plaster and water you’ll need. Once you start the mixing process, you do not want to go back and adjust quantities. To determine the amount you need, estimate the volume in cubic inches then divide by 231 to give gallons or by 58 to give quarts. Deduct 20% to allow for the volume of plaster, then refer to the table.
6. Add plaster to water.
Slowly sift the plaster onto the surface of the water. Do not dump the plaster or toss it in by handfuls. Adding the plaster shouldn’t take more than 3 minutes.
7. Soak the plaster. Allow the plaster to soak for 1-2 minutes maximum.
The soaking allows each plaster crystal to be completely surrounded by water and it removes air from the mix. Small batches require less soaking than large batches. If the soaking time is too short, it may contribute to pinholes; and if it is too long, it will contribute to fast set times, early stiffening and gritty mold surfaces.
8. How to mix the plaster.
Small batches of plaster can be mixed by hand. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream. Break down lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates-longer mixing times produce tighter and less-absorptive molds.
9. Pouring the plaster.
After mixing, tap the bucket on a hard surface to release trapped air. Pour the plaster carefully. Wherever possible, pour plaster carefully into the deepest area so the slurry flows evenly across the surface of the mold. Once the mold is poured, tap the table with a rubber mallet to vibrate the mold and release more air bubbles.
10. Drying plaster.
When plaster sets, it heats up because of a chemical reaction. When it has cooled, it is safe to remove the cottles or forms-about 45 minutes to an hour after pouring. Molds must be dry before use. Drying molds properly promotes good strength development, uniform absorption and reduced efflorescence. Dry molds evenly. Don’t set them near a kiln where one side is exposed to excessive heat or the relative humidity is near zero. Place them on racks in a relatively dry location away from drafts.
Water to Plaster Mixing Chart
This table is based on USG(r) No. 1 Pottery Plaster mixed to a consistency of 73 (73 parts water to 100 parts plaster) recommended for most studio applications. Excessive water yields a more porous but more brittle mold, and less water means a very dense, hard mold that will not absorb water.
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Are moulds the easy way out? - Sophi Sherwin Suttor
There are many ways of making pots. They range from thrown to hand built and everything in between. There is a hierarchy of respect attributed to making skills. Perhaps at the bottom end is ceramics made by using moulds. Many people who haven’t used moulds think using a mould needs no skills especially if the mould is purchased from a catalogue or off the shelf. Custom moulds attract a higher level of respect as they are thought to engage the potter at a higher level.
Moulds can be one piece producing ’flatbacks’. moulds can have one or more pieces. They often don’t have seam marks.
Most pots come out with seam marks that need to be cleaned off. This requires patience, an eye for detail and sensitive persistence.
Initially cutting back seam mark with a potter’s knife
Wiping with a damp sponge and a
final reduction with a sable like brush and water.
There are many other techniques, suggestions welcome
The clay may break at anytime when using these methods and you must start again. Reclaiming slip is very time consuming. It can alter its chemical composition and make it unusable.
Every mould is unique. Repeated use will show how each mould likes to be treated such as more time or less time and many other variables.
Where to start
I buy new and second-hand moulds. I look for the moulds to be in good condition. I like moulds where the joins are in easy to clean places like the edge of a plate and are quick to clean. I assemble the mould. I place bands on the mould. This is essential. I favour large elastic bands. These are expensive. I also like bands cut from tyre inner tubes cheap and effective. One day I hope to have every mould fitted with their own bands. I don’t like the woven straps because I can’t seem to get them tight enough. I am waiting for someone to invent straps made from elastic and Velcro.
Multi-piece mould showing use of bands made from inner tubes and a mold showing use of commercially available rubber band
Not enough bands or poorly place bands may allow the mould to split open and empty slip onto the floor/table. What a mess. I do a lot of pouring with the mould in one of those soft plastic baskets like buckets found in hardware or gardening shops. You can save the pot by quickly pushing the mould back together, adding bands and adding more slip to the mould Best to have two people working together on this or any large mould particularly if the mould stands vertically to be.
I make a lot of slip cast pottery I use rubber bands to hold the molds together. I get some persistent leakage from some isolated places of the molds. This is a waste and makes a lot of mess. I have found a solution. After I put the mold pieces together and before pouring slip I paint the outside joints with slip WHERE THE LEAKS ARE. This seals the leaking joint and stops leaks. In time you will know the places where the leaks are. This makes it easier and neater and reduces leaking to almost nil.
5 Litre bottle of slip
I work with Walkers white earthenware slip. I buy it in the 5 litre bottle because I can manage it. I am in a wheelchair. I make a lot of choices so I can go on potting. When I get the slip home, I sieve it at 800 mesh This cleans out any foreign bodies. I add about ½ cup of water to make it the right consistency for my practice. Be careful adding water as it can undo the the properties of slip. Sieving the slip gives it a beautiful creamy consistency
I return the slip to the five litre bottle using a funnel. I write a number on the bottle, so I know which one I am using. I am very good at making a giant mess with slip. I work at confining the slip. I use big funnels for slip transfers from bucket to bottles and back. I use a jug to pour moulds I buy the jugs from a teashop. They have a big capacity. They have a screw down lid. This means you can seal the jug and go and do something else. The slip remains active and it is easy to shake the jug for the slip to be the right consistency
Jugs used for pouring slip
I measure the time the slip is in the mould. At first, I start on 20 min. I adjust up and down until I get the right thickness of clay lining the mold. I then write this time on the mould. I also write the name of the mould. It helps to find them quickly. The more you pour a mould in quick succession the longer the slip needs to sit in the mould as the plaster absorbs more and more water as it becomes less efficient.
After your preferred time has elapsed you empty the moulds. Here is where you must make some choices. Used slip can be disposed of thoughtfully or into reclaiming. Used slip can go into a bottle of its own with the name written on the bottle. Or it can go back in the bottle with the unused slip. Each choice has consequences. Time and experiments will dictate your choices. After you have poured the excess slip out of the mould leave the mould for2 hours for the slip to become leather hard. If the mould is hard to separate it is not ready to open.
I have made slip from scratch starting with a big bag of powdered slip and a plastic garbage bin. You need to carefully measure all the ingredients. This can get very technical. I mix with an electric blunger. My capacity to make a mess increases dramatically.
Here are the instructions to mix your own slip
So, weighing up all the variables I buy my slip in 5 litre bottles. I have just started to buy 10 litre buckets. In the past I couldn’t get the lid off. I talked it over with the manufacturer. He changed the lids. I can get them off now.
In my situation I have an assistant. It makes life a lot easier and mess a lot less
Items made in my moulds are forever changed by alteration, addition, use of decals, cutting and pasting etc. This is done in the eternal search for new and profound meaning. It is this part of the process that deserves more respect in the ceramic world
So are moulds the easy way out in ceramics?
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I have used magic water in the past mixed with my walkers white earthenware slip. It is good but sometimes it wont cooperate. Today I wanted to join and no magic slip made up
The best time to join is when all the pieces have the same moisture content. This is often very hard to achieve. One way is to put all the pieces in a plastic bag and seal it Leave it on a shelf overnight or longer and some how like magic they settle with the same moisture content. This works very well in slab construction. I learnt this by accident when I couldn't put a slab pot together. I put the bits in plastic and decided to forget it. I got over it and all the bits were the same and went together like magic when I went back
Today I had to join. They were little birds on a tile base The parts were getting hard I used the following technique
Using a small brush put water on the approximate target
Using a small brush put some slip on top
Repeat on item to be joined
Place the two pieces together with firm but gentle pressure
Now they should be joined
So easy, so simple
Keep looking at your joins If you see any cracks paint with your slip
Recently I saw the exhibition Pieces of Shaun Hayes of Canberra (Often found in the Walkers Ceramics shop) See his virtuoso joining at his exhibition at The Stanley St Gallery
I am using One I made. It works really well
Clay Damp Boxes - My new best friend
Meagan Chaney Gumpert - Monday, March 21, 2011
Anyone who has worked with clay knows there is a lot of process involved. Certain things need to be done at certain times. But what do you do if your time and the clay’s time aren’t in sync?Well, I’ve used Damp Boxes and kept clay wet for up to a year in these babies. And I’m finding this method is perfect for the brief intervals of time I’m able to sneak out to the studio. It’s a cheep and easy solution.
In a nutshell, it works because of the way plaster absorbs water. If plaster is dryer than clay, it will draw moisture from the clay. If the plaster has been soaked I water, then it keeps the clay from drying out and maintains a humid atmosphere within the box.
I cut these circles out in early October before Cooper was born. When I finally got around to turning them into Minis, it was the end of January but the clay was still perfect to work with!
Here’s how to make a Damp Box:
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I am having some warping in the earthenware clay. I am experimenting with some hand building clay but no distinctly different results yet. I am starting to pack the big kiln but it is so cold drying is slow. I bring them into the central heating to give a good start
i am going to try some midfire porcelain. Just because I can as a contrast to the thicker earthenware.
its cold here in Canberra. Minus 4 in the morning makes the floor tiles icy. I have bought a small cafe type heater. It helps. I can work in the afternoons. I often fire up the kiln to 100 degree so or make sure the pots are dry
New plates Simple but beautiful. White earthenware with rolled texture and commercial glazes. Made on a hump mold No flat base a reference to plates being dynamic They are rocker plates. Placed on a trivet they become hover plates'.
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