What is slip casting? And how to do slip clay?

What is slip casting? To make our tableware collections, we use white porcelain. But as you may know, there are many ways to create ceramics: wheel throwing, hand building, casting, and many more! At MOCA Tableware, we love working with moulds, probably because we’re designers and not trained ceramists. We love drawing shapes and volumes, and with moulds, we can reproduce our designs exactly the way we intended.

The technique we mostly use is called slip casting, which is usually done with plaster moulds. And for that, first, we need something called slip clay!

So, what is slip clay?

Slip clay, or in our case, slip porcelain, is just liquid porcelain. We pour it into our plaster moulds. The plaster slowly absorbs water from the slip, creating a thin layer of porcelain on the inside of the mould.

You can buy ready-made buckets of slip clay or porcelain, but we prefer to make it and mix it ourselves. It’s cheaper and it gives us control over the mix. We start with bags of porcelain powder and mix them with water and a few additives in our workshop. A cheap way to do large quantities is to do it in a huge bucket and use a concrete mixer.

Each batch of slip clay needs to reach a specific density: a weight per liter. For us, it’s about 1.78 kg per liter, which means we mix 25 kg of porcelain powder with around 10 liters of water. It’s important to check this ratio every time you use it, because if water evaporates or anything goes off, it can change how thick the porcelain layer becomes in the mould. That means you could end up with the same shape, but different wall thicknesses. We also add agents to our mix to make the slip more fluid and help it release easily from the mould.

A nice thing with slip porcelain: if it dries out or a casting fails, you can rehydrate it! Just mix it again with water and your release agent.

The tricky part of slip porcelain?

Pouring and unmoulding the porcelain piece. Porcelain has shape memory, unlike many other clays. So if you make a dent or apply pressure to the piece while it’s still soft, it might never fully recover. It will probably even deform again during firing at high temperatures.

Some people pour porcelain inside the mould while it rotates on a wheel: it helps press the slip more evenly against the plaster and reduces deformation.

Also, when you pour out the excess slip from the mould, it can cause uneven thickness. 

Keep in mind too: plaster absorbs water, so you may need to refill the mould with more slip during the process. And you can’t use the same mould many times a day: it will be full of water. They need to rest too (and dry)!

If you want to try the slip casting technique:

Start simple first: buy a ready-made plaster mould (you can find them in ceramic shops) and a small bucket of slip clay. 

Once you are done with the slip casting, the piece will shrink slightly. You can then remove it carefully from the mould, just let your ceramic piece dry completely, and fire it!

Enjoy the process! If you want to know more, we advise you to have a look at our Moca cups process! And if you like slip clay, you can also check our dripping research on the Studio Joachim-Morineau website!