Slip Casting Questions

[Hilary Jean]

Hello all!

My partner and I want to slip cast cups, I have a bit of an idea when it comes to ceramics (thanks mum) but have some questions :) 

We have found a clay that already has a speckle in it - can we use it to make slip for casting? 

If the clay cannot be made into slip - is there something we could add to plain slip to make a speckled look (that's food safe)?

Can we add pigment to clay/slip that already has a speckle? Would this interfere with the chemistry?

I know the other option is to use glaze to achieve a speckled look but we would prefer that it came from the clay. (the speckled glaze I have used in the past is glossy and we want a matte finish)

I've attached a picture of the look we want to achieve with slip casting - I hope this helps

Thanks everyone xx

[hitchmss]

You could make a slip casting body from your current clay body, but it would be much easier if you could buy your clay body in dry powder form. Now, this being said, will not work with all clay bodies. Clay bodies that have larger particulates, especially sand/grog are not great for slip casting, so if your body is in this category, Id look into buying a commercially prepared slip, or finding a slip recipe.

The other thing, is that slip casting clay bodies need to be deflocculated to cast properly; a deflocculant is added to the wet slip, which allows the particles to build in even layers across the surface of your molds, without it, you'll get chunky clumps in your castings, and it will be a much weaker casting. The other thing is that slip casting bodies are different from slip recipes in that they are equally as fluid as each other, but a slip casting body actually has less water in it; the deflocculants essentially make the water "wetter"; thus if you tried to slake down your premixed clay body in water, youd have to remove a LOT of the water so that you have the proper viscosity.

Quite honestly, unless you're prepared to do some testing, a fair bit of trial and error, it would be much easier to buy a commercially prepared slip, that already has all the leg work done for you, and find a glaze which provides the surface you seek.

In regards to adding colorants to your casting slips; I have seen folks drizzle colored slips (not necessarily casting slip) into their molds, allow that colored layer to firm for a few minutes, and then fill with their regular casting slip to make their finished object. Manganese is often used in "speckled" mid fire clay bodies, however I dont know if the colorants would mess with the chemistry of the casting slip; at least, I can say that with any high amounts of concentrations it will. Using this same theory of coloring the surface of your mold with the speckled look you want, and then backing it up with your casting slip, you could likely use your clay body, thinned down, and sieved, to pour into the mold to "color" and then fill with your casting slip after. As long as your casting slip and clay body are close in their composition I do not think you'd have many technical issues with fit.

Do you have molds made already? Slip casting molds aren't super difficult to make, but they do take a bit of basic knowledge about mold making, and especially if you plan on casting those handles in place. Handles can be cast in place, but your mold making needs to be spot on so you can easily release the casting from the mold without ripping the handle off, thus most folks will cast the handle in a separate mold, and attach using your casting slip as your "adhesive" slip. If you can, keep your molds to two parts; if you cast the handle in place, it will likely be at least 4 parts.

[Mark C.]

Getting that seckile in slip cast bodies is a bit more troublesome as heaving items like the speckle will like to drop out unless they are very light. They also need to  be in suspesion evenly so say all the spots are all in one spot vs evenly distributed. My suggetsion is to talk to a few clay companies about this idea and see what comes back. You did not metion temp for fired products. The speckle seems so much easier  to put on in final glaze on top of a slip body not in a slip body .

See if you can find a commical slip body with speckle.This also may require lots of testing

In terms of using your throwing body for slip work I doubt that will be an easy task as they are so different.

Call whoever makes your clay body and ask about that formula being made into a slip body. see what they say as they have experince with that clay body.

[oldlady]

in addition to the above, see the advertising in Ceramics Monthly magazine for mold makers who can help you with the design and implementation of the master mold.  lots of experienced slip casters out there wherever you are in the world.  go visit and learn, pay for a class or lesson in how it is done.   mom might have done some mold pouring, i did a little, too but there is a lot more info out there today.  look for it.

[Hilary Jean]

Thank you all for your responses!

Our back up idea is to use ready-made slip and speckle glaze (as you suggested)

I have found a glaze selection by Mayco called Stroke and Coat. It has really great colours and speckle finish. Is it possible to dip this glaze rather than paint it on? I have painted on glaze before but I saw I missed a few spots - oops. 

https://www.potclays.co.uk/studio/products/8509/pink-a-boo-speckled#responsiveTabs 

Can anyone recommend a glaze brand that you can dip?

Thanks everyone xx

[hitchmss]

9 hours ago, Hilary Jean said:

Can anyone recommend a glaze brand that you can dip?

Pretty much all glazes can be applied in numerous methods; brush, dip, pour, spray, etc. However, you may need to thin them down to get the proper thickness, without getting an overabundance of glaze. Most glazes will be mixed to a viscosity of 1.45-1.5; some like to be applied thicker, some thinner, just depends on the surface. Some glazes that are supposed to go on thick, may look better to you applied thin. Most glazes, applied TOO thickly will either run/crawl off the pot, and some applied TOO thin will look terrible, and leave you with a surface that might not look/feel like it has been glazed. Moral of all this, is that I would experiment/test. Take the glaze you desire to use, and dip a test tile into the glaze as it has been mixed by the manufacturer. You can also take that glaze and add some water to it (without a hyrdometer, or a scale, knowing how MUCH water to add takes experience), however you might wait to see how your test tile without added water comes out first, that way you wont have a thinned down glaze you'd have to take water off of later.

Commercial glazes have a fair amount of binder/suspending agents added to them which make them much better for brushing, however, these keep the glazes "wetter" for longer; when dipping them, especially if they are straight out of the container, it may take a good few minutes for them to dry, even more so if your glaze coat is excessively thick, or your wares are thin (less porous cceramic to absorb moisture).

As with most things in ceramics....test, test,test!

[Chilly]

Hi @Hilary Jean

Everything said above is good advice.  Talk to your clay supplier.

I slipcast (hobby only) and make my slip from clay, adding Sodium Dispex to make it wetter, as @hitchmsssays above.  Works well for me.  Used to use Valentines Special Fleck Stoneware, have now switched to Valentines Royale porcelain.  

Good Luck