5 Reasons Why Your Large Batch of Glaze Doesn’t Match Your Test Batch
Same Glaze Recipe, Different Results
Has this ever happened to you? You mix up a test batch of a new glaze, dip a test tile and you like the result, so you mix up a large bucket, dip a test tile and your results look nothing like your test batch?
What’s going on? Why does this happen?
There are many reasons why the same glaze can turn out different. This article will point out some of the main reasons why you could have different results with different batches of the same glaze and some tips for maintaining consistency.
Photos: Laura Blystone. Issue: Unsolved.
1. Different Clay Bodies
It’s worth mentioning that the clay body you put your glazes on can have an effect on your fired 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.
Or, if you’re mixing a glaze from a recipe with a photo attached and your results look different than the recipe photo, it could be due to your clay body.
The biggest changes often happen when switching from light to dark clay bodies and vice versa.
Same glaze on light and dark clay bodies
2. Changes to Materials
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 obvious. The inconsistency is because most of our raw materials are mined directly from the earth and can include impurities that can be inconsistent. Sometimes they shut down mines and open new mines. Sometimes it’s because 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 change the way the glaze fires.
The inconsistency of our raw materials is something we kind of need to accept as potters. The process we use to make glazes is pretty incredible when you think about it, the fact that we’re mixing together ground up rocks that melt into coloured glass.
In order to avoid surprises due to new bags of materials, make it a habit to keep the new bag separate until you’ve tested your glazes with the new material. Once you’ve tested your glaze using the new bag of material, 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 dates I open them, just in case this info becomes relevant.
3. 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 in the kiln, 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 difference 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 a different kiln or a different firing schedule, you can often expect different results. Different sized kilns or kilns with different brick thicknesses will naturally cool at different rates, due to their ability to retain heat or not.
These bowls are glazed with the same glaze from the same bucket but fired in different sized kilns
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.
Make it a habit to use witness cones in every firing and keep track of your firing schedule and any programmed cooling cycle. Make a note or take photos as you’re loading the kiln to remember how tightly it was packed. If you have more than one kiln, mark your test tiles with something to identify which kiln they were fired in. Sometimes this info can come in handy years down the road (I have found).
4. Accuracy of Weighing Materials
When we mix glazes from scratch, one thing we need to ensure is that we’re weighing out our materials accurately. Challenges with weighing can be a result of user error, or equipment error.
User error: If we get distracted while mixing, we can forget to add a material, or add 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 better (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.
These are the different scales I’ve used over the years
When choosing a scale for mixing glazes from scratch, there are 2 terms to consider. The “capacity” and the “readability”.
Capacity is the max 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 will either have high capacity or low readability, but not both. For this reason, potters often get 2 scales. One with high capacity (5000g) for weighing large quantities and one with low readability (0.1 – 0.01g) for weighing small quantities.
The accuracy of your scale is also a consideration. Accuracy means how close it 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 every time you mix glazes.
Checking the accuracy of my cheap kitchen scale with calibration weights
Sometimes we need to just do our best with what we have!
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, both the accuracy and readability of your scale is very important.
If you have a recipe that calls for 1.4% cobalt, for example, and your scale has a readability of 1g, your scale 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 you multiply your 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 colourant like cobalt, this can be a big change.
Here’s an example of trying to get 0.2% cobalt into a 100g batch vs a 4000g batch.
Photos: Denna Nachlinger. Issue: Accuracy of weighing 0.2g of cobalt
The larger the batch you make, the more accurate your measurements are likely to be. If you’re 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 a result, I recommend testing a 1000g batch before going any larger.
A trick for weighing very small amounts
One trick for those small percentages of colourants 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.
5. 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
- Dip length
- Size/thickness of the piece
- Bisque porosity
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 bucket, 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.
Photos: Edward Shearer. Issue: Application thickness.
Measuring Specific Gravity
The way to find out the proportion of water to solid particles in your glaze bucket after the fact (if you didn’t measure while you were mixing) is to measure specific gravity. To measure specific gravity, you weigh a volume of glaze and then divide the weight by the volume. This will give you a number between 1 and 2. This number is your glaze’s specific gravity.
It’s not always convenient to measure the specific gravity of a small 100g test batch, which is why I recommend weighing your water to begin with. But it’s possible to measure the specific gravity of any glaze, any batch size, at any time. You don’t need to know how much glaze or water you started with.
When you make sure your specific gravity is always the same, your application thickness will also be the same – provided you’re using the same size test tile and holding them in for the same number of seconds each time. It’s the key to consistency.
Photos: Kathy Jones. Issue: Application thickness.
So those are 5 main reasons why your large batch of glaze can turn out different from your test batch. I hope this article has shed some light on any unexpected results you’ve gotten and how to prevent them in the future.
Learn to Mix Glazes from Scratch
If you’ve never mixed a glaze from scratch before and want to learn, I teach an online workshop called Glaze Mixing Essentials where I show you all the steps to mix a glaze and then test a base glaze with multiple colourants. Click the link for all the details.
Join my free community
If you love learning about and discussing glazes, I'd like to invite you to my free social learning Facebook group called Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes and fixing various glaze issues. Please join!