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Bone dry clay is very fragile and needs to be handled with care. Because it is fragile and damages easily you may find yourself wondering how to repair it. Or you may wonder what you can get away with in terms of decorating. Slip is often used for these purposes. The question is, can slip be used on bone dry clay?
When slip dries it shrinks. Therefore, generally, when it is applied to bone dry clay it flakes off or cracks. However, you can adapt it so that it works better on bone dry clay. Some suggestions include using specially made joining slips, calcined slips, or using engobe as an alternative.
Slip is clay particles suspended in water. It can have different consistencies, from thick cream to skim milk. The consistency you need will depend on what you are using it for. Bone dry clay is clay that is as dry as it can possibly be before it goes into the kiln for firing.
Slip is normally used on workable clay in its plastic state or on leather hard clay. It is used for a few different reasons. These reasons are:
- Joining pieces of clay together
- Mending pieces of unfired clay
- Coloring clay pottery
- Decorating clay pottery
Can Slip be Used on Bone Dry Clay For The Same Purposes?
Let’s look at each of these uses in turn and consider if slip can be used on bone dry clay.
Joining Pieces of Clay Together
Potter’s will often use slip to join pieces of clay together. For example, it is typically used to help attach handles to mugs. The usual method of applying slip is called score and slip.
The potter will score the surface of the two pieces to be joined with a sharp tool. Scoring creates two rough surfaces that will key into one another when put together.
Before joining, the potter paints or daub slip onto the scored surfaces. This helps the clay particles from both surfaces blend with one another when they are pressed together.
As a rule, joining clay with slip is best done when the clay still contains some moisture. Generally speaking, potters do not try to join clay with slip once it is past being leather hard.
In fact, the wetter the clay the better the chance that the join will be sound.
This is because cracks form in clay as it dries when moisture levels in the clay are uneven. As clay dries it shrinks. If one area of your pottery contains more water than the next it will shrink more.
This creates tension between the two parts of pottery. If the tension becomes too much, the pottery will crack, or the two pieces will come apart. This is why handles are at greater risk of coming of cups and pots as they dry.
Can Slip Be Used on Bone Dry Clay for Joining Pieces?
Potter’s may want to use slip on bone dry clay for a couple of reasons. Perhaps they forgot about a piece of pottery and it dried out before they joined the pieces together.
Or perhaps the handle of a mug did come off whilst it was drying. The question is, can you use slip on bone dry clay to join these pieces?
When slip is applied to bone dry clay, one part of the pottery will be much wetter than the next. As such, there is a lot of tension between the waterlogged slip and the bone dry pottery. The upshot is that the join is likely to break.
Also, the bone dry clay sucks all the moisture out of the slip very rapidly. As such slip won’t stay liquid and doesn’t create the liquid soup for clay particles to move about in.
So, generally slip is not used to join pieces of bone dry clay.
If you find yourself with two pieces of bone dry clay that need joining, you can try one of the following suggestions:
1) Re-wet Your Pieces and Join Them
There are a number of ways you can rehydrate bone dry clay. These include putting them in a damp box for a few days.
Or wrapping them in a damp soft cloth and placing them in a plastic bag. Once they are soft again, you can try to join them using slip.
2) Commercially Produced Clay Menders
There are some products on the market, such as Magic Mender and APT-11 which will enable you to join bone dry clay. These are referred to as ‘enhanced slips’ and can be used to join or mend greenware and bisque pottery.
They are applied directly to the dry greenware or bisque and create a strong bond that can be fired normally.
3) Using Your Own Supercharged Slip on Bone Dry Clay
There are a number of recipes recommended for making clay slip with extra bonding capacity. If you are joining bone dry clay, it may be worth trying one of these to see if it works. With bone dry clay, there is no guarantee that it will hold, but may be worth trying.
The method demonstrated in the video below involves mixing white vinegar with a few sheets of toilet paper. The toilet paper is mixed up into the vinegar until pulpy.
Crumbled bone dry clay is then added to the vinegar/paper mix and stirred in to form a paste. The mix sits overnight and can then be used as a very effective joining/mending slip.
Verdict:
Using regular slip on bone dry clay to join pieces is unlikely to be successful because of the moisture differential. Even if it does seem to have joined the two pieces, they are likely to break apart when being fired.
However, you can either try to rehydrate your pieces and join your clay in its workable state. Or you can use a special joining slip on bone dry clay, which is more likely to be successful.
Can Slip be Used on Bone Dry Clay for Repairing Cracks?
Sometimes clay will form cracks as it is drying out. Again, the reason for this is usually that there is uneven moisture levels in the form. These different levels put the pottery under strain, and a crack can appear on the surface to compensate.
The cracks can be quite substantial or they can be fine hairline cracks.
Using wet slip to mend a crack will almost certainly not work. Usually, the bone dry clay will suck the water out of the slip and the slip will flake off. Or the water in the slip will increase the tension in the clay and the crack will open up more.
An effective alternative to using slip on bone dry clay is to use paper clay to repair cracks.
Making paper clay is a little like the supercharged slip outlined above. However, there are some important differences, so read on….
How To Make Paper Clay
- Firstly, make some thick slip using the same clay body as your pottery.
- Then, put a few sheets of toilet paper in a bowl of water and let it break down.
- Once the toilet paper is in a mush, scoop it out of the water and squeeze the excess water out.
- Add the ball of strained pulped up paper to your thick slip and mix the two together.
- Let this mixture sit for a while until it is a sticky gloop rather than liquid.
- Score the crack on your pottery. You may need to add a little water to score it, but try to add as little as you can. It’s a good idea to score a wider area than the crack itself. The whole crack needs to be erased, otherwise, it is likely to open up again.
- Once the crack is scored, use the paper clay to pack out the pottery and repair the hole.
- An additional tip is to cover the repaired area with wax resist. This will slow down the drying process and may prevent the crack from opening out again.
Verdict:
Using regular slip on bone dry clay is not effective and likely to make cracks worse. A better alternative is to make a sticky paper clay gloop. This is essentially a very thick slip reinforced with paper fibers that makes it stronger and slows down drying.
Can Slip be Used on Bone Dry Clay to Add Color?
Slip can be used on pottery to add color. It can be applied in various different ways, including painting and dipping.
You can either buy colored slip, or you can experiment with making your own. Often potters will buy colored stains to add to their own slip. Ceramic stains are made from ceramic and metal oxides.
Alternatively, adventurous potters may experiment with making their own colored slip. You can do this by adding powered oxides to clay slip. Always use a respirator mask if you are handling powdered oxides.
Slip applied by dipping or painting is almost always applied to wet soft clay or leather hard clay. The reason for this is that wet slip shrinks as it dries. As such, when applied to bone dry clay, slip is likely to flake off.
However….
Here Are Some Suggestions:
- Pottery is often about trying things out. Some potters report that they use slip to color bone dry clay with some success. This approach involves trying it out and seeing if it works. The attitude is that some will flake off and some will stick.
- Sometimes potters will add water to the colored slip and make it very thin. Then they apply multiple layers of the thin slip on the bone dry to gradually build up the color.
- Other potters use slip that contains calcined ingredients. Calcined clay powder has been fired at a high enough temperature to make it harder and more durable. Some of the substances such as water and some carbon dioxide.
This makes it more stable and reduces its shrink rate. As such slip containing calcined ingredients can be applied with a bit more success to bone dry and bisque pottery. - There are a number of slip recipes that describe themselves as ‘Stick to Anything’, slips. STA’s for short. These might be worth checking out.
- Other potters recommend spraying bone dry clay with water before applying it to the slip. The thinking here is that you reduce the moisture differential and reduce the chance of the slip flaking off.
Two alternative ways to color clay are to use either an engobe or an underglaze. Both of these can fare better being applied to bone dry clay than colored slip.
Engobes
Engobes are sometimes described as being halfway between a slip and a glaze. They are liquids that can be painted or dipped.
Engobes contain fritted material. These are materials that have been heated to the point of becoming glassy. Engobes also have a lower clay content than slip. Therefore, they shrink less than slip. As a result, you are more likely to adhere successfully dry clay or bisque ware than slip.
Can Slip be Used on Bone Dry Clay to Decorate?
One of the ways that slip can be used to decorate greenware is slip trailing. This involves trailing quite thick slip onto the pottery using an applicator.
The slip needs to be thick enough so that it keeps some of its shape when it has been applied. However, slips of different thicknesses can be used to create different slip trailing effects.
The applicator, known as a bulb syringe is filled with slip and used to dispense the slip onto the clay.
Slip trailing involves simply dispensing the slip onto the clay. Because the slip shrinks it will tend to flake or peel of bone dry clay. Regular slip is, therefore, best applied to soft or leather hard clay.
However, you can also use a slip trailer to apply engobe. In this case, it is possible to slip trail onto bone dry clay and bisque ware too.
Final Thoughts
A lot of the time potters will simply recommend starting again rather than trying to mend bone dry clay. When you use slip on bone dry clay, you are battling against physics and chemistry. And you are more likely to encounter difficulties when you fire your ware.
However, slip can be used on bone dry clay with some of the special adaptations suggested above. Also, if you are trying something out in the spirit of experimentation, you may as well give it a go. What’s the worst that could happen?