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You may have heard alarming stories about pottery exploding in the kiln. To survive the firing process, you will need your pottery to be bone dry clay. You may be wondering what this means, and how to tell if your clay is bone dry. This article addresses just those questions.
Clay is bone dry when it has lost all the moisture that it possibly can before being fired. It is dry to touch, and whilst solid, it is very fragile. A common rule of thumb is that bone dry clay feels room temperature when held to your cheek.
What is Bone Dry Clay?
Before clay has been fired, it is referred to as ‘greenware’. When clay is greenware it goes through different stages of drying out. The bone dry stage is the final stage in the drying process before the clay goes into the kiln.
In its workable, malleable state, clay has around a 20% water content. When it is exposed to air, it starts to lose its water very rapidly.
In the open air, outside of its plastic bag, clay will start to dry out after 15-30 minutes. However, it is normally a matter of days before it is bone dry.
Clay is bone dry when it has lost all the moisture that it can lose before it is fired. The water in clay travels through the clay body until it reaches the surface of the piece. It then evaporates. Whilst clay may feel dry to touch, this does not necessarily mean that it is bone dry. Deeper layers of clay may still be holding moisture.
The clay is said to be bone dry when the moisture from the deeper layers has evaporated too.
How Dry is Bone Dry Clay?
Even clay that is considered bone dry from a potter’s perspective, will still contain some water.
The moisture level of unfired clay will level out in line with how moist the surrounding atmosphere is. So, if you are in a humid environment, the amount of residual water in the clay will be higher.
This is one of the reasons that potters will often start their kiln off with a pre-heating phase. When a kiln is preheating it is not firing the clay, rather it is drying it out.
The process of drying clay out in the kiln is called ‘water-smoking’. And the kiln is said to be ‘candling’ during the pre-heat.
Some potters state that candling should be done below the temperature of boiling water. The reason for this is that at 212F (100C) water turns to steam and expands (source). This expansion can cause the clay to crack or explode.
However, other potters argue that water smoking needs to be done just above boiling point. Their argument is that the remaining water will only evaporate out fully just above 212F.
Chemically Bonded Water in Bone Dry Clay
Even when all the moisture in the clay has evaporated during candling, it still contains water. When clay is completely dry, the free water in the clay has evaporated. However, at a molecular level, there is still water chemically bonded to the clay particles.
This water will only start to leave the clay when the kiln reaches around 660F (348C). It is at around this temperature that the clay starts to convert to its ceramic state.
When clay is bone dry it is very fragile and will crumble easily. Also, if you submerge it in water, it will dissolve and can be made back into workable clay. Once bone dry clay has been made ceramic, it becomes hard and cannot be transformed into a workable material.
Bearing all this in mind, it can sometimes be difficult to gauge if clay is actually bone dry. This leads me on to the next section, which tackles how to tell if your clay is bone dry.
How to Tell if Your Clay is Bone Dry
Bone dry clay is lighter in color than it is in its workable state. It also feels lighter in weight than it was before because it has lost a lot of water content.
In addition to this, it is quite crumbly and fragile. It can easily break if it is not handled with enough care.
However, whilst clay may look bone dry on the surface, it may still contain water underneath. The following test is therefore recommended:
The Cheek Test!
Hold your greenware to your cheek or wrist and notice the temperature. Does it feel cold, cool, or room temperature?
It is normally recommended that if your greenware feels room temperature, then it is bone dry.
Whilst this is a handy technique, it can cause a bit of anxiety. The temperature of an object isn’t just governed by its moisture content. If it’s a cold winter’s day in a chilly studio, your greenware may feel cool, even if it’s bone dry.
The reason the cheek test is recommended is that when water evaporates from greenware, it lowers the clay’s temperature. This is because evaporation uses up heat energy and the clay is left cooler.
However, factors other than evaporation can leave the clay feeling cooler. For this reason, some potters say that the clay may well be bone dry even if it’s cool.
The time that you can be certain that it is not bone dry is if the greenware is cold.
Never the less – ‘feeling cool’ is subjective. And new potters, anxious to prevent their pottery from exploding may want a bit more certainty.
If this is the case, then you may want to consider some of the following tips:
Making Sure Your Clay is Bone Dry
- Once it feels cool or temperate on your cheek, leave it an extra day to err on the safe side.
- Once your greenware is firm enough to lift without distorting, turn it over. By turning it over, you expose the underside to air too. If it is inverted like this, it is more likely to dry thoroughly throughout.
Just be careful that it is solid enough not to be damaged when it is upside down. Alternatively, you can place your clay on a drying rack. This will help the air circulate all around your ware.
- Make dummy pieces out of the same clay body. These pieces need to be the same size and thickness as your actual pottery. At different points in the drying process, you can break the dummy pieces open.
The moisture level in the dummy ware will be similar to your pottery. If your dummy piece is still dark and cold inside, you know your greenware is not bone dry.
- If you own a kiln, candle the greenware for a while before firing. Thinner pieces may only need to be candled for an hour or so. Larger, thicker pieces may benefit from being candled overnight to make sure they are bone dry.
- What if you don’t own a kiln? Perhaps you are taking your greenware to a kiln firing service. If so, then you can still do a bit of DIY candling in your home oven.
Place your greenware in the oven if it fits. Then set your oven to around 200F (93C) for a while. This will help evaporate residual moisture. Just remember to keep monitoring the situation.
With time potters get to know their greenware well and can gauge whether it is bone dry clay more easily. Eventually, you will work out how long it takes, and you will be able to adjust your drying schedule. For example, you may need to add extra time for drying in the summer months when it is more humid.