Paperslip
by Steven Goldate

Inspired by a mid 18th century Qing dynasty ‘triple’ cloisonné vase, I recently embarked on a multi-faceted project that would challenge many of my skills. In the course of this project I acquired some new skills and came up with some – I hope – novel solutions to problems that I was presented with. The focus of this article is on the main material I used – paperslip, and on slip casting in general.

Most people by now would have heard of paperclay. It has been the topic of a number of articles, e.g. by Graham Hay and Gaye Stevens and a of couple of books (notably Rosanne Gault, Paperclay). Surprisingly, when I mention to people that I am experimenting with paperslip, they nod in recognition, as if this was a material well known to them. However, I am not aware of many people having experimented with it or published anything on the topic.

So just what is paperslip? Answer: a casting slip, into which a certain amount of paper pulp has been mixed. Why use paperslip? Answer: because of the great drying and gluing qualities it offers, similar to paperclay. In the following, I would like to go into some detail on the advantages, disadvantages (if any) of paperslip and the processes connected with it.

The problem I was originally presented with was how to make an intersecting three-necked vase from clay, which I had originally seen in cloisonné. For the 40-50 cm high piece I was envisioning, casting seemed to be the only viable option. I also decided that some paper pulp added to my slip would improve it’s workability in the leatherhard to dry state. I set about coil building a model from which I made a two piece mold, that sat on a plaster base. The idea was to cast the pieces in paperslip, then cut the sides at 120 degree angles and glue them together with more slip. In reality, this turned out to be impossible to do. After cutting the sides, the neck’s weight made it sag slightly to the unsupported side. My necks had to be parallel, so I never made it to the gluing stage with these casts.

Back to the drawing board. Literally. I decided I would make a new mold from a model that already had the sides ‘pre-cut’ at the necessary 120 degrees. This way, the neck would be supported and I could glue the walls together relatively effortlessly. However, this gave rise to another problem – how to make a model, so that it’s neck wouldn’t sag? I ended up making a double-walled vessel, where the neck extended into the belly of the form, down to the base so it was self-supporting. Again, this model was coil thrown.

The next step was to make the walls. These had to not only be at an angle of 120 degrees to each other, but also perpendicular. To make sure I got these angles right, I constructed a ‘gauge’ of sorts. This was made from particleboard with the help of a right angle and compass. After cutting the model’s belly at roughly 120 degrees, I was then able to attach flat slabs fairly accurately with the help of my ‘calibrated’ gauge. I wound up with quite a satisfactory model.

With shrinkage and cracking of coils in mind, I wasted no time in pouring the plaster. I did this using a method shown to me by Kurt Weiser of Arizona State University. His method is to wait until the plaster starts to stiffen and then to ‘slop’ it on the model by hand. Weiser first pours a thin layer over the model to get a really smooth, even coat. Then, using a new batch of plaster, molds this on by hand, immediately after it has started to stiffen. The first thin layer didn’t work for me. I found it didn’t adhere to the mold sufficiently, parts sticking to the model and flaking off. I skipped this stage and got a very good surface anyway. I don’t really want to go into the intricacies of mold making here – enough has been written about that already. But I do want to talk about cottles.

What is a cottle? Answer: a cord of some sort, which is tied around a mold so it holds together when you pour the slip in. This usually works fine for small molds. But some molds, like the ones I was working on, took in the order of 30 liters (63 pints) of slip. That’s probably about 50 kilos (110 pounds) plus the weight of the mold itself we’re talking about. Having had the experience of losing slip from an insecurely fastened mold, I decided something more efficient was needed. My method consists of drilling holes through the sides of the mold, pushing bolts through and securing these with nuts. Never, ever will a mold come apart again using this method. When designing the mold, it helps to modify the design slightly, incorporating flanges to accommodate the pins. You may also want to embed some rings into the plaster for protection and strength. A good alternative to drilling the holes might be to pour the mold with some dowling in place, which could be easily removed once the plaster has hardened sufficiently. Bolts of just about any length can be obtained from a good hardware store.

Weiser had told me that it was quite possible to stick pieces of slipcast clay together without any special treatment, but for what I had in mind I was skeptical. Being a fan of paperclay, I thought paperslip would be an ideal material to work with. Paperclay gives greenware great strength, minimizes (if not eliminates) cracking and reduces weight and shrinkage. When making paperclay or paperslip, one of the first considerations is what sort of paper to use. I found that newsprint and office shreddings worked equally well for me. As far as slips go, any commercial or home made slip should work well. Keep in mind that the paper will introduce a small amount of calcium carbonate (used in paper as a filler) to the mix, which may raise the maturation temperature. However, with the quantities of paper I am suggesting to use, this shouldn’t be a problem.

Whichever paper you use, it is important to let it soak for at least a week or two. This helps to break down the fibers. Another option might be to boil the pulp, although I haven’t tried this yet. Another factor is the size of your pre-pulping paper shreds – the smaller they are, the finer the fibers of your pulp will turn out. Because we are casting, it’s better to have smaller fibers that can mix well with the slip, creating as homogenous a mix as possible. Once the paper has soaked long enough, the mixture can be worked on with a drill mixer attachment or blender. This is to further beak down the fibers. (Just down tell your partner you’re using his/her favorite $90 Braun kitchen blender for the purpose…)

After mixing the pulp sufficiently (a matter of personal judgment), it can be put in a kitchen strainer and left to drip-dry for a bit, or some of the water can be squeezed out. This is so that we don’t introduce too much extra water into our slip. On the other hand, the paper pulp should not be so dry, as to thicken the slip so that loses it’s easy pouring quality. Add about 10% by weight of medium wet paper pulp to your slip and give it a good mix with your mixer attachment. For paperslip, this amount should be enough. Paperclay can take up to as much as 50% paper content, but with slip, the casting qualities would be reduced considerably.

Don’t use new paperslip immediately. The mixing action introduces lots of air bubbles into the mix. When you pour a cast, these rise to the surface and create small bubbles in the walls and on the surface of the piece. These bubbles can eventually lead to pinholing – not good! For this reason, it’s a good idea to let fresh paperslip rest overnight. Before using it, mix it by stirring slowly it with a stick, being careful not to introduce any air. Another way of inadvertently introducing bubbles into the slip is by pouring it from a height. The splashes and splatters generated by gravity introduce – you guessed it – more air bubbles! The solution is to pour the slip down a slide of some sort, e.g. a piece of plastic pipe, available in various diameters from hardware stores. (Tip: sandpaper the edges, so they don’t damage your mold’s surface.) Apart from some of the things mentioned above, general rules of slipcasting apply.

I was quite happy with the results of my mold and cast. The next stage was to stick three sections together. I scoured the walls of three leatherhard pieces with a sawtooth blade. After brushing on fresh slip, two sides could be joined together, carefully aligning the edges and the inner ‘spine’. The same procedure was followed with the third piece, except that there were four walls to join. This is where you find out if your angles are correct! If there are small gaps here and there between the seams, these can be filled in with slightly stiffened paperslip. This technique proved itself – not a single crack appeared anywhere. The downside is that the walls can soften too much and make joining difficult. When I tried to join pieces by rewetting the walls after they were bone dry, some small cracks did appear the bisque firing. It might be possible to fill these and re-bisque the piece.

What I ended up with, was a three-necked, three chambered vase. Later, to save clay, reduce weight (not great in any case) and reduce the ‘gluing’ area, I decided to cut out the walls, leaving a one inch rim to join. In this case, I ended up with a three-necked, one-chambered object. Decorating these objects is going to take some thought and will be a challenge. As they are in a sense three independent, intersecting pieces, it would be astute to decorate each one somewhat differently, enhancing it’s individuality, in contrast to a uniform glaze. However, that may become another story altogether.

Related links:
More on Paperclay
Porcelain Paperclay
Slipping into Paperclay

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