Tired of spending hours, if not days, working on and refining the perfect ceramic form only to find a crack that seemingly developed overnight? Before you go switching out your clay body, take some time to reexamine your drying process. Drying ceramic work is often seen as an uncontrollable process or not that important. Improper drying can lead to warping which creates tension and will ultimately result in cracking.
Before delving into tips and tricks to create a suitable drying environment for your ceramic work, lets breakdown greenware shrinkage, moisture gradient, and some reasons why cracking occurs during the drying process.
What is greenware shrinkage?
Greenware shrinkage is the permanent contraction of a clay body, from its most plastic stage to bone dry. The particles of the clay body reorganize themselves to fit closer together as the water evaporates resulting in the overall appearance of a smaller form.
What is a moisture gradient?
The moisture gradient is the high and low water content areas of a clay body throughout an individual ceramic form.
Understanding the moisture gradient based on your building method:
Extrusion- The center of extruded coils will have a higher water content than the outer surface of the coils. Using an extruder can create a variety of forms and generalizing a moisture gradient for this style of building is not useful.
Wheel- These forms will have a higher water content on the outer surface of the walls, wherever water is used as a lubricant to aid in forming and shaping. The collection of water on the inner bottom of a wheel-thrown form will increase the water content in that area.
Slip cast- Forms will have a higher water content on the surface of the walls that do not touch the mold.
[Side Note: If your slip contains heavy particles or nonplastics (such as grog) that settle to the bottom of your form during the casting process, ununiformed shrinkage will occur.]
Slab- Initially, the center of a slab will have a higher water content. If the slab is placed in a mold that is absorbent, the higher water content will be on the surface of the wall that does not touch the mold. If the slab is placed in a mold that is not porous, the higher water content will be on the surface of the wall that touches the mold.
Why are greenware shrinkage and moisture gradient important?
Understanding how your ceramic form is drying can help you understand why it is cracking during the drying process. When your ceramic form is drying faster in one area than another, the contraction of the particles in faster drying areas will create tension and in the efforts to relieve that tension the form will crack.
[Side Note: If you are having problems with cracking at points of connection (such as seams or handles) uneven drying may be the cause. These connections create a weak point and are perfect for cracking to relieve tension.]
How do I dry my work?
All uncontrollable variables aside, dry your ceramic work in an environment where your clay body can dry evenly and at the same rate. The key concept to keep in mind is that the water evaporates from the outer surface of the walls by air or through absorption.
Tips and tricks that you can incorporate into your drying process:
Prop it- Propping your ceramic work on mini-stilts allows air to flow more evenly around your work.
Flip it- Similar to the propping technique, air flows more evenly around your work. Keep in mind that if you have a form that will generate tension due to poor weight distribution by flipping or has a large surface area that remains blocked from air flow, the flipping technique is not ideal.
Defend it- Lightly wrapping your form in plastic wrap or a damp cheesecloth changes the amount of air flow around your work and creates an environment where water evaporates at a slower and more even rate.
[Side Note: Using fans to dry work quickly can be considered as long as air is able to flow evenly around the form.]
Drying your work is an important process with many factors to consider. When deciding how you will approach the drying stage, evaluate your building method and the form you have created before choosing a drying process or a combination of processes that will work best for you.
Lawrence, W. G., & West, R. R. Chapter 6 – Drying. Ceramic Science for the Potter (2nded.) (pp. 82-91).
Katz, M. Lecture 9 – Working with Clay Pt.2. Introduction to Clays Online .