Kiln Wash: Protecting Your Kiln Shelves during Firing

Kiln Wash

Welcome to Hook’d Up Bar and Grill! In this monthly tech-tip, we’ll be discussing the kiln wash recipe. No tracking, no ads – just quick loading for your convenience. Kiln wash is a high-melting powder used to prevent ware from sticking to kiln shelves during firing. It also acts as a parting agent to prevent elements of fired ware from sticking to each other.

Understanding Kiln Wash

Kiln wash is a refractory powder that, when mixed with water (and gum solution), can be applied to kiln shelves. Its purpose is to keep ware and glaze drips from sticking. For example, porcelain clays tend to adhere to the kiln shelf during firing due to their melting properties. However, with a good kiln wash, this becomes a non-issue. Some clays contain soluble salts that fire to a glaze-like sheen, and they also tend to stick ware to shelves. By properly protecting the shelf with kiln wash, molten glaze can run onto it during firing and then be easily lifted off when the kiln is unloaded.

The Ingredients for Effective Kiln Wash

To ensure the effectiveness of kiln wash, it is important for the powder to be as refractory as possible, meaning it can withstand high temperatures without melting or sticking. Common kiln wash recipes often contain a high percentage of calcined alumina, which is considered the best option. Additionally, zircon can be used for its high refractoriness. These powders, when mixed with kaolin and silica, create a clay-body-like material that provides excellent protection for kiln shelves.

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Application Techniques

It is common to apply kiln wash by painting it onto refractory shelves and furniture. When using the 50:50 kaolin:silica recipe, it is best to minimize the amount of water used to maintain suspension. This ensures a smooth application. However, the refractory nature of kiln wash can make it challenging to achieve an even thickness and smoothness when it dries on-contact to porous refractory surfaces. One solution is to add CMC Gum, which slows down the drying speed and makes the kiln wash more paintable. The gum also prevents cracking during drying, allowing for the application of multiple layers if needed.

Visual Examples

Flaking kiln wash on a silicon carbide kiln shelf

Flaking Kiln Wash

In this image, you can see an example of a 50:50 kaolin:silica kiln wash recipe. While economical, this recipe tends to become excessively hard and shrink during drying and firing. To achieve a more desirable result, it is recommended to use a recipe that remains somewhat powdery to the touch. By using pure calcined alumina with a small amount of calcined kaolin (around 10%), you can minimize shrinkage. Adding 1% CMC gum creates a paintable material, although it may take some time to dry.

A running glaze has stuck to a kiln shelf. Kiln wash saves the day!

Kiln Wash

This image shows the effectiveness of a zircon-based kiln wash. Despite being applied in a thin layer, it prevents a very runny glaze from sticking to the shelf.

Kiln wash that really works. How?

Kiln Wash

The image above displays two shelves. The one on the right has a traditional kaolin:silica kiln wash recipe, which constantly flakes and sticks to the feet of the ware. In contrast, the shelf on the left is coated with L4001, a kiln wash that is perfectly even and thin. It remains refractory and doesn’t become brittle or crack. This successful kiln wash recipe includes zircon (Zircopax) mixed with calcined kaolin. The addition of Laguna gum solution further improves the drying and stabilization properties of the kiln wash.

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The foot ring on the left is plucking, the right one is not. Why?

Kiln Wash

These translucent porcelains are fired to cone 10. The foot ring on the left, made from a cone 6 body, sticks to the kiln shelf and warps during firing. The foot ring on the right, with only slight plucking, is also vitreous but doesn’t experience the same issues. By using a more refractory kiln wash, such as one based on calcined alumina or zircon, the plucking problem can be resolved.

Even with good kiln wash, plucking can sometimes occur

Kiln Wash

These marks were left by ware made from a cone 6 highly vitreous translucent porcelain. To avoid plucking, it is advisable to set such ware on a thin layer of silica sand or granular alumina.

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