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Though a successful kiln firing is dependent on many factors, one particularly valuable tool used to ensure both an artist’s work and machine is protected is kiln wash.
What is kiln wash and why do you need it, you ask?
This article will cover just that; we’ll guide you through kiln wash’s purpose, how to mix and apply it, and what you should know before using it.
Whether you’re a ceramic or glass artist, new hobbiest or professional, knowing the properties of kiln wash and the rules of its use are essential for you to make your best work and will help you avoid costly problems.
Also referred to as shelf primer, kiln wash is a liquid substance that is brushed onto kiln shelves to prevent glaze from an art piece sticking to the shelf. Though you can make your own kiln wash from one of the many recipes that exist, most people buy kiln wash pre-mixed.
Kiln wash is made of refractory substances, like silica, alumina, or zirconium, that have high melting points and don’t cause melting when combined. The exact formula is adjusted based on an artist’s intended firing temperature; silica is typically used in low- or mid-fire temperature ranges, while alumina, which is sourced from kaolin, the foundation of porcelain clay bodies, is typically used for higher temperatures since it has a higher melting point.
Zirconium oxide or zirconium silicate are used for high-fire programs because these ingredients have even higher melting point than alumina. Though higher temperature formulas always work at lower temperatures as well, they are more expensive.
Kiln wash provides a protective layer between your artwork and your kiln shelves. Though glaze is never intended to come in direct contact with kiln shelves - hence, why stilting or wiping glaze off the bottom of a project is important - sometimes it will unexpectedly run, spit, drip, or melt onto the shelf.
Because kiln shelves are typically made of hard ceramic material, glaze will stick to them if one of the aforementioned situations occurs. Therefore, kiln wash is used to ensure your pieces won’t stick to the shelves if something goes awry during firing.
It is also used as a preventative measure in case of bloating or melting clay bodies, or in case a piece tips over, all of which could similarly lead to an unfortunate fusion between a piece and a kiln shelf.
Kiln wash should be used during every glaze firing. If it isn’t, you’ll risk not only the difficult job of removing a stuck piece, but the task of grinding all the glaze off the kiln shelf. This is both hard work and damaging to the smooth surface of your shelf.
Any glaze you miss during the grinding, along with what seeped into the pores of the shelf, will continue to eat away at the shelf and weaken it.
If you are using a pre-made kiln wash, it typically will come in a powder form. Here are the steps you should take to mix and apply it:
Fire new kiln shelves dry. Before even mixing your kiln wash, if you are using brand new kiln shelves, it is recommended you first fire them once in an empty kiln. This initial step will drive out any organic material from manufacturing and transport. Now they are ready to have kiln wash applied.
Mix a small amount of wash in a bowl. Mix some of the powder with water until it has the consistency of skim milk.
Apply thin coats. Using a wide paintbrush, utility brush, roller, or even sprayer, apply a thin layer of wash to the kiln shelf. If the coat is too thick, it could cause more cracking and peeling. Also, be careful to avoid getting kiln wash on the edge of the shelf; it can flake off during firing and onto your glazed pieces below. Many artists sponge off a half-inch of kiln wash around the perimeter to ensure it won’t go over the edge.
Apply at least 3 coats of kiln wash. To ensure that there is enough protection between your ware and the kiln shelves, you should apply three full coats of kiln wash. Let each coat dry thoroughly before applying the next one - allow about 60 minutes per layer. Some artists even chose to fire between the application of each coat, building it up during bisque firings or adding additional layers during glaze firings.
Scrape any wash that flakes off. If you notice that your kiln wash is flaking, remove any loose pieces with a paint scraper and reapply. Similarly, if any glaze drips onto your shelf, use the paint scraper to remove it, clean up any loose flakes around it, and then dab some kiln wash on the bare spot.
It’s also important to note that you should only apply kiln wash to the top surface of your shelves. If you would like to flip your shelves over, you need to scrape all the kiln wash off the side you’ve already used to prevent flakes from falling onto the pots below.
If you’ve used kiln wash, you should have no trouble detaching a piece from a kiln shelf where glaze has dripped. When you lift it, the kiln wash in that area will come up with it, but you can grind it off the bottom of your pot if it otherwise looks good. You can fill the spot on the shelf back in with more kiln wash, but over time the shelf will become uneven from multiple applications.
Continuing to fire on a rough shelf can cause pots to get hung-up and crack, warp, or break. Therefore, after several firings, you need to clean off the remaining kiln wash and start fresh with new layers.
Here are some important things to keep in mind when cleaning kiln wash off your shelves:
Before loading, scrape loose, flaky kiln wash off: Examine your shelves carefully before firing, and be sure to remove any kiln wash remnants from a previous firing with a sturdy metal scraper or wire brush.
Wear safety gear: Glasses or a face shield along with a good dust mask is suggested, and when scraping, brushing, or grinding shelves, it is safest to do the work outdoors.
Remove any accumulation of glaze residue: Minor glaze drips can be chipped off with a hammer and chisel, but never hold the chisel vertically against the shelf. For more serious glaze runs, you will have to grind the glaze and kiln wash off.
On occasion, kiln wash is unnecessary. Here are the situations in which you will not need to use this otherwise essential tool:
The first firing of your ware, known as a bisque firing, doesn’t include glazes, so poses fewer threats to both your work and kiln shelves. While kiln wash is not necessary for this stage, it is still recommended due to the fact that expansion and contraction can cause pot “feet” to get stuck, some porcelain clays have a habit of sticking, and the risk of clay meltdown is always a possibility.
Some glass kiln shelves are made from fiber, allowing them to be made larger and flatter than most ceramic shelves. Kiln wash should not be applied to fiber shelves, as they can absorb water and will therefore be destroyed. If you have a kiln with fiber shelves, use kiln shelf paper instead (more on this below).
Glaze does not stick to nitride bonded silicon carbide, so if you own kiln shelves made of this material, it is typically unnecessary to apply kiln wash. However, if you are firing finicky materials - namely porcelain - which can become glaze-like under certain firing circumstances, you might want to use kiln wash to prevent your clay bodies from sticking, especially when using a gas kiln.
It is recommended that you also apply kiln wash to the brick on the kiln floor. If any glaze drips onto the floor of the kiln, the wash will prevents the bricks from being penetrated and destroyed by the liquid.
However, if there are kiln elements in the bottom of your kiln, do not apply kiln wash to it; the ingredients in kiln wash will aggressively destroy heating elements, so in general you should steer clear of applying wash to the side walls or any other areas elements might be located. To reduce the chance of contact between kiln wash flakes or chips and heating elements, be sure to keep your kiln clean by occasional vacuuming.
While silica is the preferred refractory ingredient in many kiln washes, it is not a good choice for salt or soda firings. Silica is a glass-former, meaning that if a lot of glaze drips onto a shelf coated with kiln wash, the glaze can melt the silica in the kiln wash and form a new glaze on the shelf. This is particularly likely in an environment where sodium oxide is introduced atmospherically - such as in salt and soda firings - so the best kiln washes for such firings are ones not made with silica. You can mix your own kiln wash with a salt-specific formula that uses a different refractory (like kaolin or alumina oxide), and use “wadding,” a thicker version of kiln wash that more closely resembles bread dough, to place under your pieces.
An alternative to kiln wash is kiln paper, which performs the same function as kiln wash but instead comes in the form of a thin, paper-resembling sheet that disintegrates after firing. It is quicker and easier to use, but it is more expensive. Typically it is preferred by glass artists who don’t want the brush strokes from kiln wash application to show through their transparent pieces or otherwise alter the texture of their work’s bottom.
We hope that this guide has helped you to better understand the what, why, and how of kiln wash use, and will inform your kiln firing programs from here on out. Please don’t hesitate to contact us if you have any further questions!
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