How to Make a Good Kiln Wash to Protect Your Kiln Shelves
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Different Kinds of Kiln Shelves
Before I talk about kiln wash, I want to talk about kiln shelves.
The kiln shelves that I’ll be referring to in this article are high alumina kiln shelves. Alumina is highly refractory, meaning it melts at a super high temperature (cone 42) which makes it a good material for making kiln shelves.
High alumina kiln shelves are the beige coloured shelves that are sold with most electric kilns nowadays. They are the cheapest kiln shelves you can buy and they work well but are prone to warping after many high-temperature firings.
There are other kiln shelves out there that are lighter, don’t warp, are more expensive and don’t require kiln wash. Nitride-bonded-silicon-carbide shelves, for example, don’t require kiln wash because glaze won’t stick to them (magic!). If you have this kind of kiln shelf, you don’t necessarily need to worry about kiln wash.
But if you have high alumina kiln shelves like I do, using kiln wash is highly recommended.
High alumina kiln shelf without kiln wash
Kiln Wash Saves Our Kiln Shelves
Kiln wash is a material that you can paint onto your kiln shelves. It looks very similar to a glaze when being applied. It acts as a barrier to prevent unexpected glaze runs or drips from ruining your shelves.
Kiln shelves are made of a hard material that is similar to fired clay. If you don’t use kiln wash, any glaze that comes into contact with your kiln shelf gets absorbed and becomes permanently fused to it.
Glaze drips on unprotected kiln shelves
The only way to get the glaze off a bare shelf is to grind down the part of the shelf that the glaze has stuck to. This is a process that takes time, elbow grease and usually leaves your shelves gouged, bumpy and uneven.
We don’t want bumpy, uneven shelves because this can cause our pots to warp during the glaze firing. We want our shelves to be nice and flat.
When we do use kiln wash and our glaze runs, the kiln wash prevents the glaze from coming into contact with the shelf. Our pots may be ruined but we’ve saved our shelves! This is a win because kiln shelves aren’t cheap.
The reason kiln wash is so effective is because it’s very high in alumina (like our shelves). Remember, alumina is a refractory material that won’t melt at our firing temperatures. So the kiln wash won’t fuse to our shelves.
Since kiln wash can’t fuse to the shelf like a glaze can, the layer of kiln wash can be removed from the shelf. Think of it like a drop cloth when painting a room. Any paint drips are absorbed by the drop cloth and then the drop cloth can be removed, leaving a drip-free floor underneath.
The drop cloth protects the floor, the kiln wash protects the shelves.
So a glaze that runs or spits (or was not wiped from the bottom!) will stick to the kiln wash, not the kiln shelf. Then the kiln wash can be removed from the shelf, saving the shelf from potential permanent scarring.
Kiln Wash Saves Our Pots
Kiln wash can also save our runny glazed pots from cracking when removing them from the shelf. If a piece is fused to a bare shelf by glaze, something will have to break in order to remove it (usually the pot is sacrificed, saving the shelf).
But since the kiln wash allows the pot and glaze drips to be released intact (if the drip is small enough) it may be possible to grind the drip off the bottom of the pot and still have a fully functioning piece of pottery.
The other thing kiln wash prevents is our clay sticking to our kiln shelves. Some porcelains, due to their high silica content, become so fluxed (melted) during the firing that they have the potential to stick to an unprotected kiln shelf.
This can lead to “plucking” where small bits or splinters of clay stay fused to the shelf, breaking free from the piece itself. It’s a disappointing way to ruin a beautiful piece of pottery.
A good kiln wash should prevent plucking, although a potter recently informed me that she is experiencing porcelain plucking with her commercial kiln wash.
Since we don’t know what a commercial kiln wash is made of, I can’t say exactly why this is happening. It’s possible that the kiln wash is high in silica like porcelain, and they’re both fluxing and fusing together.
What I do know is that we solved our plucking issue with the kiln wash recipe I share down below.
We were experiencing plucking with porcelain at the studio where I work. We use small discs of clay as “cookies” so any glaze drips will land on the cookie, which we can throw away. This prevents a lot of kiln shelf maintenance.
The cookies were originally made of bare clay with no kiln wash. Once I noticed the plucking, I decided to apply kiln wash to the cookies as well as the kiln shelves. Since we started kiln washing the cookies, we haven’t experienced any more plucking.
To read more about plucking and see some photos, Digital Fire has more info https://digitalfire.com/4sight/glossary/glossary_plucking.html
How to Make a Good Kiln Wash
You can often buy kiln wash in powder form from your ceramic supplier. You just add water and it’s good to go.
You can also make your own kiln wash from ingredients that you likely have in your glaze lab. It’s cheaper to make your own and you can also tailor the kiln wash recipe to suit your needs.
A good kiln wash is not too flaky or powdery but comes free from the kiln shelf with little effort. It requires minimal maintenance when your glazes are behaving.
I found that the commercial kiln wash I tried was way too flaky. I don’t know what kind it was, I inherited it with a bunch of other glaze materials.
A flaky kiln wash is messy, has to be re-applied often, and has the potential to land on your glaze during the firing.
Cracking and flaking is caused by the kiln wash materials shrinking. This can be prevented by designing your kiln wash for minimal shrinkage, which I have done in the recipe I share below.
The other thing you don’t want is a kiln wash that is too hard to remove from your kiln shelves. This can happen if your kiln wash contains flux, or if you’re firing in a flux-filled atmosphere (wood, salt, soda firings).
The best kiln wash stays stuck until you try to remove it, and then it scrapes off easily.
What Kiln Wash is Made Of
Most kiln wash recipes out there contain some combination of three ingredients: Alumina, EPK and/or Silica.
Some use just one or two of the three and some use various combinations of all three. Some recipes contain other materials but these are the 3 I have used.
Let’s look at these 3 ingredients and what properties they each bring to the kiln wash.
Alumina: highly refractory, expensive ($7.34/kg), powdery, doesn’t melt, doesn’t shrink
EPK: highly refractory, cheap ($2.23/kg), bonds other particles together, doesn’t melt, high shrinkage
Silica: highly refractory, very cheap ($1.96/kg), can melt if it comes into contact with fluxes, doesn’t shrink
(costs are from my Canadian supplier in 2019)
Sue’s Kiln Wash Recipe
I’ve created a kiln wash recipe that takes all of these properties into account and uses each of them to their greatest advantage.
I measure each material by volume, or by the scoop. I don’t bother weighing since it doesn’t need to be exact.
2 scoops Alumina Hydrate
1 scoop EPK
1 scoop Calcined EPK
1 scoop Silica
Flocculate with Epsom Salts solution
Here’s a link to my kiln wash recipe on glazy.org https://glazy.org/recipes/49756
Now I’ll walk you through how I came up with my kiln wash recipe by describing each material in more detail.
Start With Alumina
We could make kiln wash out of pure Alumina. We can sprinkle alumina hydrate onto our kiln shelves, or we can mix it with water and paint it on. Alumina is all that’s required as a barrier between our runny glazes and our kiln shelves.
Using pure alumina means you have a powder that does not bond with the kiln shelf. It remains a powder during the firing and can be poured back into a container and reused.
The benefit of using pure alumina is that your kiln shelves remain bare and kiln wash free so they can be flipped over, firing to firing, preventing warpage.
Using alumina powder also gives your pots the freedom to move during the firing, without friction. All pottery shrinks during the glaze firing and if there is too much friction between the pot and the kiln shelf while the pot is shrinking, the pot may crack.
Using a powder that can move eliminates the friction, kind of like ball bearings. This is especially helpful when firing extra wide pieces that have a large point of contact with the kiln shelf.
The issue with pure alumina is that you have to be careful that none of the alumina powder falls onto any of your glazed pieces. If so, it will become a permanently embedded blemish in your fired glaze.
The fact that I work at a busy pottery studio where kilns are being loaded and unloaded daily by several different people means using a loose powder in our kilns could be messy and disastrous. It’s important that our kiln wash sticks to the shelves and stays there.
This is where the EPK comes in.
Add Some EPK
By adding EPK (or any kaolin) to the alumina, we still have a highly refractory kiln wash, but the kaolin helps to bond the alumina particles to each other, and to the kiln shelf.
Note that bonding is not the same as melting and fusing. The EPK helps the particles stick together like clay sticks together, but they don’t stick permanently to the kiln shelf as a glaze would.
The EPK also helps to keep the kiln wash in suspension, so it mixes well and doesn’t hard-pan in the bucket.
I often see kiln wash recipes that are 50% alumina and 50% EPK. But there’s a problem with this recipe.
One drawback of using kaolin is that it shrinks. Kaolin is clay and we all know that clay shrinks when it dries and when it’s fired. This is true for kaolin in a glaze recipe and kaolin in a kiln wash recipe.
Too much kaolin causes the kiln wash to shrink too much, crack and flake off. If kiln wash flakes off during the firing, it can land in your pots and cause blemishes in your glazes.
Cracking, flaking kiln wash
A glaze recipe with over 25% EPK can have problems with shrinking, cracking and crawling. Same with kiln wash. So this 50% alumina 50% EPK recipe has way too much EPK, causing way too much shrinkage.
The solution to this is to substitute some of the EPK with Calcined EPK. I do this with glazes, and I do it with kiln wash.
Substitute Half the EPK with Calcined EPK
Calcined EPK is EPK that has been fired high enough to remove the chemically bound water molecules, essentially pre-shrinking it. I wrote a whole article on how and why to calcine EPK here.
So I substitute half the EPK with Calcined EPK. This greatly reduces the shrinkage but provides enough clay for the kiln wash to bond with itself and the kiln shelves.
Now I have a 4 part recipe:
50% Alumina, 25% EPK and 25% Calcined EPK.
This could also be written as:
2 parts Alumina, 1 part EPK and 1 part Calcined EPK
because I don’t weigh the materials, I just go by the scoop.
Add Some Silica as a Cheap, Non-Shrinking Filler
The recipe I had at this point should be just fine, but I took it a step further.
I had seen kiln wash recipes that call for silica so I tried adding 1 part silica. By adding 1 part silica, I now had a 5 part recipe.
40% Alumina, 20% EPK, 20% Calcined EPK and 20% Silica
2 parts Alumina, 1 part EPK, 1 part Calcined EPK, and 1 part Silica.
Adding 1 part silica only reduced each of the other materials by a small amount. Since silica is very inexpensive, I see it as a cheap, non-shrinking filler.
It reduced my EPK from 25% to 20%, which would reduce the overall shrinkage of the kiln wash.
I also read that the silica may act as a hardener, causing the kiln wash to hold up to more wear and tear before crumbling and needing to be replaced. I can’t confirm or deny this claim, but I do know that our kiln wash lasts many, many firings before we have to scrape it off and start fresh.
I’ve been using this kiln wash recipe for 5 years in a high volume studio that fires 5-6 kiln loads/week and it has been amazing! Eventually, it does start to crack and need replacing but we can go several months before any maintenance is required if there are no glaze disasters.
This shelf has been fired over and over and the kiln wash is still smooth and intact
A warning about Silica in kiln wash:
Since silica is a glass former, it has the potential to melt into glass if it comes into contact with fluxes. Atmospheric firings that introduce salt or soda require a silica-free kiln wash.
If sodium (a flux) in the atmosphere comes into contact with the silica in the kiln wash, it can melt and turn into a glassy layer on your kiln shelves. This would be very problematic and would defeat the whole purpose of the kiln wash.
I use my kiln wash recipe in electric and gas firings and it works great. If you’re firing in other atmospheres, I would recommend leaving the silica out and going with the 4 part recipe 50% Alumina, 25% EPK and 25% Calcined EPK
Water and Epsom Salts
All of the above ingredients need to be mixed with water and a high-speed mixer like a hand blender. If it’s lumpy, or I don’t have access to a high-speed mixer, I’ll sieve my kiln wash so it’s smooth.
I add enough water so that it’s the consistency of milk (quite thin) and then I flocculate with saturated Epsom salts solution.
The Epsom salts solution thickens the kiln wash, improves suspension, prevents hard-panning, and improves brushability by causing it to stay wet for longer.
There’s nothing worse than touching your kiln wash brush to the kiln shelf and it dries instantly.
A flocculated glaze or kiln wash stays wet longer, allowing you to brush on an even coat before it dries.
How to Apply Kiln Wash
Kiln wash can be applied by paintbrush, roller or by spraying. I’ve only tried the brushing method. A friend of mine uses a roller and swears it’s life-changing, but I have yet to go out and buy a roller to try it.
Make sure your kiln wash is well mixed before applying. You want it to be homogenized. I stir with my brush each time I dip back into the kiln wash.
When brushing kiln wash, I always use a large wet sponge to dampen the kiln shelf first. Pre-wetting the kiln shelf prevents the water in the kiln wash from being absorbed into the shelf too quickly. We want the kiln wash to stay wet until we have an even coat.
I use a wide, soft-bristled brush that will hold as much kiln wash at a time as possible. Dip the brush into the kiln wash and without rubbing the bristles on the edge of the bucket to remove excess, start painting the shelf.
As soon as the brush is dry, go back for more. Apply the kiln wash very liberally with long strokes. The key is to get a nice even coat. Using many small brushstrokes can make for an uneven layer.
Once the shelf is covered, I allow it to dry to the touch and then add another layer.
When finished, wipe any drips off the edges of your shelves. If you leave them, they could flake off and land in your glazed pots during the firing.
And make sure you don’t get any kiln wash on the bottoms of your shelves either. Tops only!
Always make sure your kiln wash is completely dry before firing. It will change from a darker colour to light when it’s dry. If the kiln wash is still damp and you put it in the kiln, you have more chance of it flaking and not adhering properly to the shelf.
They may take a couple of days to dry. To speed up drying, use a fan or put near a hot kiln.
If possible, put your freshly kiln washed shelves through a bisque firing before a glaze firing. Bisque firings are generally slower so any leftover moisture has time to escape. This isn’t always possible but if I have the option, that’s what I do.
After the first firing, check your shelves. If you can see the shelf through the kiln wash, add another layer. You want the kiln wash to be thick enough to prevent glaze from seeping through.
Kiln Shelf Maintenance
There’s a lot I could say about kiln shelf maintenance. In the future, I’ll write another detailed article all about it.
But I will leave you with one kiln shelf maintenance tip.
Over time, your kiln wash will start to flake and chip off. You may have craters of un-kiln-washed areas. Especially if your glazes have been running.
Craters in kiln wash from removing glaze drips
The tendency is to fill in those holes with more kiln wash. In my experience, this leads to a very messy, bumpy surface that will crack again and can lead to warped pots. We want flat, smooth kiln shelves.
I’m big on keeping up with kiln shelf maintenance and not letting them get out of hand. When a shelf needs attention, I don’t just fill in the craters.
Here’s what I do instead:
I first take a grinding block and (wearing a respirator) I gently grind off the top layer of kiln wash. I just take off the layer that wants to come off. I don’t fight with it or try to get right down to the bare shelf. I make large sweeps with the grinding block to get the surface as smooth and even as possible.
Kiln shelf grinding block – image from Greenbarn.com catalogue
There may still be kiln wash left on the shelf. The idea is to remove the high points so the remaining layer is somewhat level.
Then I apply 2 layers of kiln wash to the entire shelf, following the instructions above. By applying the kiln wash to the whole shelf, you have a fresh, smooth, even layer.
If a kiln shelf gets really bad, I’ll use more aggressive grinding methods. But for touch-ups, this is the method I use. It’s fast, effective and long-lasting.
Storing Kiln Shelves
And the last thing I’ll say about kiln wash is this:
Make sure you store your kiln shelves in such a way that the bottoms of your kiln shelves aren’t in contact with the kiln washed side of another shelf. Store them so the kiln washed sides face each other and the non-kiln washed sides face each other.
You don’t want bits of kiln wash from the top of one shelf to get stuck to the bottom of another kiln shelf and then fall onto your glazed pots during the firing.
So I store my kiln shelves in an alternating pattern. Bottoms touching bottoms and tops touching tops.
There, that’s all I have to say about kiln wash, for now. Do you have any questions or kiln wash tips to share? Please leave a comment below. I’d love to hear from you.
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Awesome article!!! You are such an amazing wealth of knowledge. Thank you Sue!!!
Hi Sue! Thanks for all the info!!
Thank you so much Sue, very useful !
Dear Sue thank you for the article. I coat my kiln shelves with kaolin only and I have the problem of flaking. I look forward to experiment with your recipe.
Great, I hope your kiln wash improves!
Thank you for this advice Sue – I’ve been firing my kilns by myself for the last 20 years and this is the first time I’ve seen the advice about storing shelves washed face to washed face!
Oh cool! Yeah, I learned to do that in school and now I’m a stickler about it!
Over 40 years ago i stop using alumina and changed to Titanium Dioxide. Equal parts TiO2 and Kaolin by volume. It never hard pans, applies easily and resists glaze wonderfully. After each firing we wet sponge the shelves. every 10 firings we light sand and redcoat. It has never got to the stage of flaking.
I’ve never heard of using titanium dioxide in place of alumina. That’s very interesting. Thanks for the tip!
Thank you Sue for this most informative pdf on kiln shelves. I am in the process of trying various kiln shelf washes but so far they all flake. I really appreciate your explanation of each ingredient included, it all makes sense finally.
Again my thanks for your generous sharing.
Thanks for the great article! The only question I have is about the saturated Epson salt solution. What’s the ratios on this and about how much additional of this solution would I add? Thank you.
I would add about a tablespoon of saturated epsom salt solution to a gallon of kiln wash and then mix with an immersion blender and see if it noticeably thickens. If so, then that’s enough. If it’s still thin and watery, add another tablespoon and blend again. It’s not crucial to get it “exactly right”. The purpose of the Epsom salts is to flocculate the kiln wash, which keeps it suspended in the bucket and prevents hard-panning and it also helps it to stay wet for longer when brushing onto shelves.
Apologies if I’m repeating myself at all, I didn’t go back and read what I wrote in the article 🙂
Why you don’t take molochite? It’s calcined Kaolin: Al2032Si02 same for both.
But, I have an other question: I use this Kiln wash from John Britt :
40 hydrate alumine, 30 calcined kaolin or molochite,30 kaolin, 1 Feldspath.
But after few month, it’s mold and smelt, can I put 1tsp of bleach?
Hope you can understand my English ! Have a nice day
I don’t use molochite because I have calcined kaolin in my studio and have never purchased molochite. You could use molochite if that’s what you have. And yes, I’ve used a small amount of bleach in smelly clay buckets before so I’m sure you could use it with smelly kiln wash too.
Hi Sue, can I ‘calcine’ some EPK myself by firing it or should I buy some already calcined?
Hi Susan, Yes you can calcine your own EPK by putting in the bisque firing. I have a blog post about that here https://suemcleodceramics.com/how-to-convert-kaolin-to-calcined-kaolin/
What Silica mesh size do you suggest for your kiln wash or does it matter?
I have access to Silica #200.
I’ve honestly only tried one mesh size (325) so I don’t know for sure, but I don’t think it would matter in kiln wash.
Hi. I recently need to make my own kiln wash for the first time.
So I cleaned my shelves off w hammer and chisel.
Then I took a John Britt recipe (I had not found your page yet, sorry)
50 Alumina, 25 Kaolin and 25 Quatrz. Disaster struck and all my cups and plates glued to the kiln shelf. To be exact the kilnwash turned a glossy white green color.
So I asked my old ceramic teacher for help and she told me to use
25 Alumina, 50 Kaolin and 25 Quartz. I even bisque fired these, all good. Then the same thing happened again the shelves turned a glossy white green and all the ceramics got ruined. So her answer was I must have mixed in the wrong ingredients. I think not. But remixed and only painted one shelf this time, and put 1 cup on it, and … the area around the cup was bluish and glossy, cup stuck and the rest of the shelf a hard white surface that I have no clue how to get off.
I don’t have Calcinated Kaolin. I know your wash is slightly different, but it really doesn’t explain what is happening. I don’t have much kiln experience as I have only been firing on my own a year.
Hi Marketa, I’m afraid it does sound like maybe one of your materials is mislabeled. What you’re seeing is the kiln wash fluxing, which could happen if you added something containing sodium, potassium, lithium or boron. Maybe you have a frit that’s mislabeled, or gerstley borate mislabeled as EPK? I would maybe put a very small amount of each of the materials you tried by themselves on a tile or in a clay dish, and see which one turns glassy in the glaze firing. Neither of them should turn glassy if they are what they say they are. John Britt’s recipe should work just fine, as should your teacher’s recipe.
Only if it was mislabeled from the shop. All materials in original packaging. Thank you
I would call the shop and tell them what you’re experiencing because it doesn’t make sense. Silica, alumina and epk have very high melting temperatures, way above what our kilns are capable of.
I cleaned off the old wash yesterday. today I put 2 coats of wash from Standard. The weird thing is that after its dry, it is powdery. Not sure what i did wrong.
Any ideas. Never had this happen.
Hello! I purchased a used kiln that came with shelves and the shelves had wash on both sides. Do I just sand one side down? Thanks!
How can I apply kiln wash on my perforated kiln shelves? Should I apply it in the holes too?!
Thank you Sue for sharing so much of your knowledge. I have taken both your glazing courses and highly recommend them to anyone who will listen! Its a great feeling to not only make the pots but to make the glazes too… And now with this article I am confident my newly kiln washed shelves will last.
Thanks Lesley, so nice to hear my courses and blog are helping people!
I was having issues with plucking, though i had no idea it had a name! A friend I told about this problem sent me your blog post on kiln wash that mentioned plucking and I was so happy to find this issue addressed. I made your kiln wash and literally grounds and chipped the old wash off by hand (w/o damage to shelves thankfully) and applied your kiln wash. It was BEAUTIFUL and still is a few firings later. And no more plucking! I am so grateful. I am now also taking your Mastering Glaze Consistency Course. Thank you so much for sharing your knowledge and experience.
I’m so glad my article helped you make a good kiln wash and prevent that pesky plucking! Hope you’re enjoying Mastering Glaze Consistency 🙂
Hi Lisa- did you add the epson salt solution & if so what is the ratio of water & epson salt before you add it to the rest. Thank you…
I use a similar kiln wash recipe, have been using for 30+ years without any issues. I use 1 scoop alumina hydrate, 1/2 scoop EPK and 1/2 scoop calcined EPK. I stopped using silica in my kiln wash years ago as I did experience some plucking with some porcelain bodies and it’s just safer when cleaning to avoid additional silica.
I apply the kiln wash, that is consistency of whole milk, using a cheap 4″ paint roller to get a more even surface as applying with brush can lead to slightly uneven surface and cause thinner ware to warp.
I have since bought Advancer nitride bonded siC shelves and apply a light dusting of alumina to prevent plucking.
What is the best way to store shelves to keep them and the kiln wash undamaged when not in use?
I store my kiln shelves upright and leaning against each other, against a wall. Kind of like plates drying in a dish rack. Kiln washed sides face each other and unwashed sides face each other to prevent the transfer of kiln wash flakes to the bottom of another shelf. If you have to store them flat, I wouldn’t stack more than 3-4 high because the weight could cause the bottom ones to crack.