What is casting slip?

An Etsy customer recently asked what “slip” is (because it is listed under my material section for most of my items) and then, a day or so later, my husband asked what slip casting is, even though he see’s me pouring slip all the time – I suppose he just didn’t know the name that is attached to the process .  It’s another occasion where, because I know what something is, I assume others do too.  So, I though I’d explain this mystery substance that is the basis for most of my products.

Slip is a liquid clay and can be any type of clay from earthenware to porcelain clay.  Water is added to help achieve a liquid state but there is not as much water in slip as you would think.  Actually, slip contains only a little more water than clay that you would use on a wheel or use for slab building.  The magic ingredient that makes clay castable is deflocculant, solutions that contain sodium ions (+) that act to keep the clay particles from packing together and settling.  Of course there are other things that go into making a workable slip, ratios of different raw materials, etc.  But that’s the gist of it – clay, water, deflocculant.  It seems pretty straight forward until, like me, you add too much deflocculant and end up with slip that performs horribly, either gums up or gets so liquid it settles out.  So, on my creative quest, I had to call on my science background and start measuring things like specific gravity and viscosity.  After leaving college chemistry classes, I never thought I’d use a triple beam scale again, let alone own one.  But I do and I use it every week to measure the weight of slip and divide it by the weight of an equal amount of water to determine the slip’s specific gravity.  I’m shooting for a range of 1.74 – 1.76.  Then I take my stoppered Erlenmyer flask and time the amount how long takes for 500 ml of slip to flow out.  It’s all very scientific and important, hee hee.  But really, all this measuring has helped me get a handle on mixing and working with slip.  Thankfully, there is wonderful information in books and online which has helped immensely.  These are a couple of my favorite books The Essential Guide to Mold Making & Slip Casting (A Lark Ceramics Book) by Andrew Martin and Slipcasting (Ceramics Handbooks) by Sasha Wardell.  These deal mostly with porcelain and not stoneware, which is what I use.  Thankfully, Jaun from Laguna Clay has been helpful along the way.

Each day I pour slip, I mix it up well (because it thickens as it sits) and then fill up my molds.  Water from the slip travels through the plaster molds via capillary action, leaving behind a coating of slip on the inside of the mold.  This coating, the cast, gets thicker the longer the slip sits in the mold.  The amount of time that slip is left in the molds varies depending on what it is – a small dipping dish takes about 30 minutes and a stein mug about 60 minutes to get to the right thickness.  When the cast reaches the proper thickness, the slip is dumped out of mold.  The coating of slip that remains inside the mold is allowed to dry until the piece can be handled – this us usually a few hours.

Then the mold is taken apart and the cast removed.

At this point the spare clay from the mold opening is trimmed, attachments (like mug handles) are added

and and the seam lines (ridges of clay where two mold pieces come together) are removed.  The piece is then let to dry a little more and final trimming is done.  Once the piece is bone dry, it is further sponged to remove any residual traces of seam or other blemishes and to smooth out edges and remove any dust.

It is then ready for firing, decorating, glazing……

I love the process – I was attracted to it immediately.  I thought it would be simple but turned out to be more challenging as I got more into it.  But I’ve meat every challenge with enthusiasm to learn more and am thankful for my achievements.  I look forward to continued learning on the subject of slip.  ‘Cause, just when you think you’ve got something down, the Universe throws a curve – just so things don’t get boring.