I spent this past week at a work shop with Harriet Caslin at Santa Fe Clay entitled Plaster Model & Mold Making, Concept to Finish. I chose Ms. Caslin's workshop because of her absolutely stunning lighting fixtures, which you can find here! She also makes beautiful tableware, so be sure to check out her shop.
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| Harriet Caslin's most popular Birdcage pendants. Aren't they gorgeous?!? You can purchase them here! |
But, based on the title and brief description of the workshop, I have to admit that I really had no idea what to expect, other than that I was (hopefully) going to learn how to make molds.
Well, it turned out that, in addition to making molds, I learned something completely new. Something unexpected. Something that I had never even heard of! and that is.....
...sledging plaster (or plaster sledging).
If you look "sledging" up in the dictionary, you generally get two definitions, both of which are British, and neither of which have anything to do with plaster (or pottery). It seems that sledging is usually defined as either:
1. a British term for sledding, -or-
2. a term used in cricket to describe the practice whereby players seek to gain an advantage by verbally insulting an opposing player.
Okay...........
With a bit more research, you find that sledging plaster has been around a long time. It has been used for centuries, for example, to create decorative moldings in castles and residences, and is a great way to restore sections that match the original molding in vintage homes and buildings.
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| Source: ‘Model-making Basics’ – modelling and shaping |
For pottery, plaster sledging is a way to create a shape that will last a long time and that can be made into a mold to use to slip cast into pottery. Since your original form is plaster, rather than clay, it can be made into many molds, and if your molds chip, or break, new molds can be made from your original shape. Which is great for production potters! .....which I'm not.
And don't want to be.
But, there are a few things that I can use this for.
1. Items that I make a fair number of AND are tedious to make.
-AND-
2. Travel mugs (because I have a hard time throwing large travel mugs to the correct height and diameter of the silicone travel mug lid every time).
So, I decided to sledge a travel mug form. I wanted it to be large (most American commuters like their coffee, and I can throw small travel mugs fairly well). I need the finished diameter to fit a silicone travel mug lid. I also wanted a simple, basic shape, so I can individualize each mug and to have the option of attaching a handle. And, it needs to fit into a car's cup holder.
The standard silicone travel travel mug lid fits a rim with a diameter of about 3.5", and the casting slip we were using had a shrinkage rate of about 12%. So I designed a shape with a diameter of about 4". (I'm not very mathematically inclined, so if my math is off, please don't leave a long comment below explaining my error. I'll figure it out soon enough, I'm sure.)
***On the other hand, I'd love almost any other kind of comment just so I know someone is reading this!***
SLEDGING
For the purpose of creating a ceramic slip casting mold, the sledging process is as follows:
1. Create a base that is the diameter of the item you want to cast and a tool that is the shape of the profile you want your ceramic object (in this case a travel mug) to have. Make sure to put notches/divets in the base.
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| My finished base and sledging tool (left). The original shape I cut from aluminum (right). The base was creating by pouring plaster into a 4" round shape. We used metal flashing to create the round shape. A small coin was used to carve out the divets.
To make my tool, I made a "blade" by cutting aluminum into the shape I wanted the side of my travel mug to be with tin snips. I then anchored it in place with a bit of clay and created a reservoir around it with clay. I then poured plaster into the reservoir. While the plaster was still soft (a bit like cottage cheese), I scraped it away from around the blade, thereby creating a handle that supported the tall blade and would move nicely around the round base.
The photos below show Harriet Caslin demonstrating this for the workshop. At left, she begins forming a reservoir around her blade. At right, she removes plaster from around her blade after pouring it into her completed reservoir.
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*Important Note: Make sure you cover all surfaces with some type of plaster release or soap if using surfaces that plaster sticks to. Plaster DOES stick to itself! It does NOT stick to clay and most flexible/bendable surfaces/molds.
2. Pour a cylinder of plaster and use your tool to sledge it! This is harder than it looks, so start with a simple shape. You only have about 5 minutes (or less) during which the plaster is the correct consistency to do this. It took me 3 tries to get this right! And, truth be told, Harriet did it for me the 3rd time, because I was feeling a bit dejected and frustrated.
Here are some photos of Harriet demonstrating this process:
1. Glue your divited base firmly to a rotating work surface. Wrap it with a thin amount of clay and then with metal flashing.
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2. Tie your flashing tightly *Harriet uses cut bicycle tire inner tubes) and tape &/or clip the middle and top of the seam. You may want to place clay around the outside of the base of the flashing as well to prevent leaking. Measure and pour your plaster (see appendix below).
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Here's a link to a hyper-lapse video I posted on Instagram of Harriet demo-ing the sledging process pictured above: Plaster Sledging Demo.
Let your shape dry a bit more, but when it starts to warm up, use some water and sand paper to smooth it out on all sides. Now you have a perfect shape that is exactly (albeit a bit larger) like what you will ultimately have in clay!
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| My final plaster travel mug shape. Now I'm ready to make a mold (or 2, or 3+). Before you say that this is not too exciting, remember my criteria: a basic shape so that I can individualize each slip cast travel mug by adding shapes, painting designs, or using sgraffito, decals, or screen printing on it. |
Mold Making
Crack Molds
Harriet taught us how to make crack molds, which are SUPER SIMPLE 4-part molds for symmetric objects. I don't have any photos, of my travel mug mold making process, but I also sledged a 2nd shape for a mug with a rounded bottom. I have a few photos of that mold making process.
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| Concept illustration of my mug shape. |
In order to create the mold:
1) Create a base for your shape's bottom. I turned my sledged mug shape upside down, soaped the top (really the bottom), wrapped clay around the top (er, bottom), and embedded flashing tightly in the clay around the base. I then poured about 1.5-2" of plaster and, while it was still soft enough to carve, removed the flashing and carved a groove around it about half way up. It is like creating a "hat" for your upside-down sledged shape, but this "hat" will eventually be the center bottom/base of your crack mold. Leave the "hat" "stuck" to your sledged shape and proceed to step 2.
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| My sledged shape and it's "hat." |
2. Place your shape (as shown-still upside down) on a plaster base that is at least 2" larger than your shape in diameter (on all sides). My mug above has a 4" diameter at its largest point, so I had to pour a plaster disk at least 8" in diameter earlier in the day. Place your shape and "hat" as is (upside down) on the center of your base disk. Use a small amount of clay to fill any gaps between the bottom of your shape and the plaster base. This is important because plaster will find a way to slip into the tiniest of crevices! Use a metal scraping tool and small sponge to remove as much excess clay as possible, so that the clay is just between your plaster shape and the plaster base. SOAP all the plaster that you can see!!!!!!! Soap all the plaster that you can see! Soap all the plaster that you can see!
3. Wrap the metal flashing around your base. Tie tightly and tape seam. If you'd like, place clay around the bottom outside seam to prevent leaking. Measure enough plaster to come to the top of your cap/hat, but not over. (Deduct the volume of your shape + cap/hat from the total volume of the cylinder to the top of the cap/hat.) It is always better to underestimate the size of your shape, so that you have more plaster, rather than less plaster. Pour your plaster. Stop pouring when you reach the top of your hat/cap.
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5. Your mold will look like a white cylinder. Let it sit for about 24 hours. Isn't it pretty?!? You can see the hat/cap at the top center in this photo (below). This will actually be the bottom of the mold.
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6. Remove the large round base from the bottom of your mold. (this is really the top of your mold, the side you will pour the slip into.)
7. Now it's time to crack it open.
You can find 3 short, time-lapse video segments on cracking a mold open here, but basically the process is as follows:
- Divide your mold into 3rds, lay your mold on its side, and use a saw to saw down about 1/2" vertically at each 3rd. These are the "lines" along which your mold will crack open. (1st short video at link above)
- Stand your mold vertically again. Place 2 steel plates into one crack with the saw lines. Now place a third plate between the 2 plates. Use a mallet to gently tap the 3rd plate in between the 2, widening the sawed slit, until a crack forms from your sawed slit to your hat/cap. Do this for all 3 saw grooves. (2nd short video at link above)
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| Mark the top of your mold across the cracks with 1, 2, and 3 scratch lines, so that you can put it back together correctly. You can see my scratches faintly in the photo above. |
- Gently pull apart your mold. Pull the top cap/hat off, too. (3rd short video above) Turn your mold over. When you strap/tie the four pieces in the bottom photo below together, your mold is ready for slip casting your shape!
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| If you have soaped everything well, your original sledged form/shape should pop right away. You can follow the crack-mold making process outlined above to make as many molds as you want and then wrap it and store it away for safe keeping. The 4 pieces pictured above are all you need to slip cast your form for years to come! |
2-Part Molds
Harriet also showed us how to make a a 2-part mold. I made one for a simple optional handle for my travel mug. She explained that it's important when slip casting a cup or mug to also slip cast the handle because regular clay may shrink at different rates due to deflocullants added to slips. If you'd like to read more about deflocullation with regards to slip casting, click here.
This handle may look simple, or boring, but there is a bit to think about when adding a handle to a travel mug: it must be attached low enough so that the silicone lid fits over the top of the mug and high enough so that the mug fits into a car's cup holder. So there really isn't a lot of room to get fancy!
It was the last day of the workshop, so I didn't have time to use this to cast with, or to clean it up much, but I look forward to trying it!
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| This photo of the handle mold next to the 1st cast of the travel mug with the lid set on top gives some idea of what I'm going for. (Minus any decoration and glaze, of course) |
I still need to make a mold of a handle for my smaller mug. I'm hoping to make something like this:
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| Sketch of mug with rounded bottom concept |
Pouring ~ Slip Casting
It was the last day of the workshop, so I only had time to test my travel mug mold with one round of slip. Here is a photo montage:
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This is not my mold, but I poured my mold similar to Harriet's demo mold in photo 1, above, let it sit for 12 minutes, emptied the excess slip into the bucket, and turned it over to drain, much like Harriet's demo mold in photo 2, above.
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| When the clay began to pull away from the mold and felt leather hard, I removed the mug from the mold. You can see that it the exact shape as my original sledged plaster shape, but a bit smaller due to shrinkage. It is the 1st test pouring for this mold, and therefore will not be fired, nor will the 2nd or 3rd pourings, as I need to test for timing/thickness (this is too thin) and make sure there is no more plaster residue. But, I am pretty darn happy! |
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| This photo shows the top of the slip cast travel mug standing upside down on top of the top of the sledged plaster travel mug shape 24 hours after casting. The clay is now bone dry. You can see that the crack mold seam is very small, so future castings will require very little clean up, and that the clay has shrunk a bit more. |
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In this photo I have placed a silicone travel mug lid on top of the bone dry slip cast travel mug. You can see that the outer edge of the lid almost lines up with the outer edge of the mug. This concerns me because, if I were going to fire this piece, it would still have 2 firings to go through: a bisque firing and a glaze firing, during which a lot of shrinkage can take place. I really hope that after all my hard work, this travel mug won't shrink down to a diameter that is too small for the lid when finished. But the only way to know will be to cast a few more mugs, and to put them through the firings. Keeping my fingers crossed!Appendix 1 - Calculating Volume and Plaster Amounts
The formula for calculating the volume of a cylinder is:
V
=
π
r
2
h
π=3.14, 2=radius, h=height
Harriet is British, so we did this in centimeters and liters, which is actually quite simple.
Example:
To pour a cylinder to sledge that has a base of 4"/10cm and that is 8"/20cm high, you would 1st calculate the value of
r2. If the diameter of the base is 10cm, then the radius is 5cm. 5cm squared is 5X5, or 25cm.
So, plugging those values into the equation above you get:
3.14 X 25 X 20 = 1570cm3
To convert that to liters, simply move the decimal to left 3 places (ain't metrics grand?). 1.57l or 1.6liters Now multiply that by .8 to get the liters of water you want (80%):
1.6 X .8 = 1.3 liters of water.
Now multiply that number by 1.3 to get the liters of plaster you want: 1.3 X1.3 = 1.7 liters of plaster. To mix the plaster, add the 1.3l of water to a flexible plastic bucket. Gently sift in 1.7l of plaster. Mis with your hands, gently agitating your fingers along the bottom to get the bubbles to rise to the surface. Scoop as many bubbles as you can out onto a scrap surface (sheet of paper, plastic tarp/bag, etc.). When the plaster is still runny, but starting to thicken, pour into your mold. Follow directions above after that.
The formula for calculating the volume of a cube is:
Base (b) X Height (h) X Length (l)
I used this for making the 2 part mold for my travel mug handle.
Here is an excellent video on how to make a 2-part mold. The only difference is that I made 2 pour-holes, and this video shows how to make one pour-hole. But making 2 pour-holes is just a matter of adding 2 clay spouts instead of 1 (that will make sense after you watch the video). Appendix 1 - References and Sources for Sledging Plaster for CeramicsVideo: Ed Bentley Sledges a Smooth Oval Shape Video: Using a Pottery Wheel to Sledge Plaster Video: Hirotaka Tobimatsu Shapes Plaster on the Wheel Article: How to Get the Perfect Profile by Sledging Plaster Course: Plaster Mould Making for Slip Cast Ceramics with Ed Bentley Plaster Workshops in Guldagergaard, Denmark
See more of my ceramics at FunNFiber and follow me on social media:
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