Have you ever mixed a test batch of a new glaze, fired it, and liked the result so much that you mixed a large bucket, only to discover that the fired results look nothing like your test batch? Why does this happen?

Same Glaze Recipe, Different Results

There are many reasons why the same glaze can turn out different when batching up to make a larger quantity (1). Below are five main reasons for variable results with different batches of the same glaze and some tips for maintaining consistency.

A. Different Clay Bodies

The clay body you use can affect your fired glaze results. So, if you’re trying to match a glaze you made years ago and you’ve switched clay bodies since then, this could be a factor. The biggest changes often happen when switching from light to dark clay bodies and vice versa (2).

1 Left tile top and bottom: Correctly mixed glazes in 1000g-batches. 1 Right tile top and bottom: Incorrectly mixed glazes scaled up to 6000g batches. (Top and bottom glazes are the same base with different colorants.) Photos: Courtesy of a member of Sue McLeod’s Facebook group. 2 A single glaze recipe shown fired on a light clay body (left) and a dark clay body (right).

B. Changes to Materials

The inconsistency of our raw materials is something we need to accept as potters. The process we use to make glazes is pretty incredible when you think about it—we’re mixing together ground-up rocks that melt into colored glass. Every time you open a new bag of a material, there’s the potential that you’ll end up with a change in your glaze. Sometimes the change is subtle, sometimes it is obvious. The inconsistency is because most of our raw materials are mined directly from the earth and can include impurities that may vary. Sometimes companies shut down mines and open new ones. Sometimes one bag of material came from one part of the mine and another bag came from another part. Every material in a glaze recipe has an effect on the glaze’s appearance and melting temperature. Any changes to the composition of the materials can alter the way the glaze fires.

In order to avoid surprises due to any differences in new bags of materials, keep the new bag separate until you’ve tested your glazes with the new bag of material. After successful testing, then you can safely dump it into the bin containing the previous bag. I like to keep track of the dates I purchase bags of materials and the dates that I open them, just in case this information becomes relevant.

C. Firing Schedule and Cooling Cycle

The amount of time your glazes spend in a melted state in the kiln, both on the way up to the top temperature and on the way back down to room temperature, can have an effect on your fired results. When glazes melt, they turn into a liquid on the surface of your pots. While glazes are in the liquid state, molecules have the ability to move around and interact/react with each other, and are subject to gravity.

A glaze that reaches its top temperature really quickly can turn out very different than a glaze that took a long time to get to the same temperature. The change can be even more drastic if there’s a big variation in the length of the cooling cycle. In both cases, it’s the amount of time the glaze is in its liquid state that’s significant. This generally occurs above 1500°F (800°C).

If you’re using another kiln or an alternate firing schedule, you can often expect different results. Kilns that are different sizes or that don’t have the same brick thickness will naturally cool at different rates, depending on how well they retain heat (3).

3 Four bowls with the same glaze. The pink ones were fired in a large kiln (slower natural cool) and the yellow ones were fired in a small kiln (faster natural cool).

If your kiln is packed tightly with pots, it will take longer to cool than when your kiln only has a few pots on each shelf and more empty space. These are all things to take into consideration.

Use witness cones in every firing and keep track of the firing schedule and programmed cooling cycle. Make notes or take photos when loading the kiln to remember how tightly it was packed. If you have more than one kiln, mark test tiles to identify which kiln they were fired in. 

D. Accuracy of Weighing Materials

When mixing glazes from scratch, we need to ensure that we’re weighing out our materials accurately. Challenges with weighing can be a result of user error or equipment error.

User error: Getting distracted while mixing can result in forgetting to add a material or adding the same material twice. I always make it a habit to weigh the material, then check it off the batching sheet right before dumping it in the bucket. If I wait until it’s in the bucket to check it off, I’m more likely to forget and move on to the next material. This method works for my brain, but you might use another method to make sure you’re checking off each material as it’s added.

Equipment error: Not all scales are created equal, and your scale might be more accurate at weighing large quantities than it is at weighing small quantities. A really good scale is often pricey and most people will buy a cheaper scale when they start mixing glazes and then upgrade when they realize they want more accuracy. The best scales I’ve used have been Ohaus brand, both digital and triple beam.

E. Water Content and Application Thickness

A very common reason for inconsistent glaze results is the glaze application thickness. Application thickness can be affected by a variety of things: water content and specific gravity; the length of time a piece is submerged in the glaze; the size and thickness of the piece; bisque porosity; and flocculation.

But the most common reason for different glaze-application thickness when scaling up from a test batch to a large batch is a change in water content. The proportion of water to solid particles in your glaze bucket is going to affect the thickness of your glaze application. More water in a glaze will generally give you a thinner application.

Weighing the amount of water that you add to your glaze is the best way to make sure you don’t add too much. I usually start with 75% of the dry batch weight of water in the bucket and add my dry materials to the water. So for a 100g test, I would start with 75g of water in the cup and then add the dry materials. For a 5000g batch, I’ll start with 3750g of water in the bucket.

Once the glaze is mixed and sieved, more water can be added to get to a good dipping consistency. If you weigh any water added to your test cup, then you can write it on your recipe and make sure you add the same proportion of water when you make a larger batch.

I hope this article has explained any unexpected results experienced when scaling up a test batch of glaze and how to prevent them in the future.

Scale Considerations

When choosing a scale for mixing glazes from scratch, there are two terms to consider: “capacity” and “readability.”

Capacity is the maximum weight the scale is capable of weighing. You could have a scale with a capacity of 500g or 5000g, for example.

Readability is the smallest increment of measurement on the display. You could have a readability of 1g, 0.1g or 0.01g, for example.

Most scales in a modest budget range will either have high capacity or low readability, but not both. For this reason, potters often get two scales, one with high capacity (5000g) for weighing large quantities and one with low readability (0.1–0.01g) for weighing small quantities.

The scale’s accuracy is also a consideration. Accuracy means how close the scale comes to the actual value—does 10g weigh 10g? You can buy calibration weights to make sure your scale is accurate, although even a slightly less-than-accurate scale can give you consistency as long as you’re always using the same scale. But watch out if you’re trying to weigh really small quantities. Even scales that can be fairly accurate for larger quantities can be very inconsistent when trying to weigh under 10g or so.

When mixing test-sized batches of glaze and then scaling up to a larger batch, the scale’s accuracy and readability are both very important. If you have a recipe that calls for 1.4% cobalt, for example, and your scale has a readability of 1g, it will only tell you if you have 1g or 2g. Mixing a 100g test batch is very likely to give you inaccurate results. There’s no way you can tell if you have 0.9g, 1.0g, 1.1g, 1.2g, etc. You just know you’re somewhere around 1g. You may like your results, but when you scale up, you’ll be mixing a slightly different recipe. Say you mix a 100g batch that’s supposed to contain 1.4% cobalt and instead of 1.4g, you actually end up with 1.1g. This difference might seem insignificant, but when multiplying the batch by 10 or 100, your measurements get more accurate. The large batch of glaze is closer to the intended recipe, but different from your test batch. With a powerful colorant like cobalt, this can be a big change (5).

The larger the glaze batch, the more accurate your measurements are likely to be. When mixing glaze batches less than 500g, I recommend having a scale with 0.1g or 0.01g readability. If your scale has a readability of 1g, which is the most common, I don’t recommend mixing less than 200g test batches. If you do test a small quantity and like the result, test a 1000g batch before going any larger.

One trick for those small percentages of colorants is to weigh out more than you need and then visually divide it up until you get the weight you want. For example, if I need 2.5g of something, I can weigh out 10g (I weigh onto a folded piece of paper) and then visually cut the 10g in half twice to get 4 piles of 2.5g. This can often be more accurate than trying to guess where the halfway point between 2g and 3g is on a digital scale.

the author Sue McLeod has been a studio potter since 2010 and worked as the ceramics studio technician at a community studio in Victoria, British Columbia, Canada, for 6 years. She has now transitioned to working from home full time, researching glazes in her home studio and teaching three online glaze courses: Glaze Mixing Essentials, Mastering Glaze Consistency, and The Art of Glaze Chemistry. She also runs a large, free glaze-support community on Facebook titled “Understanding Glazes with Sue.” Read more ceramics articles and get info about registering for her glaze courses at suemcleodceramics.com.

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