Troubleshooting Commercial Underglazes
Want to participate in this study? Step one is gathering data.
Download and print this form to keep track of your underglaze results in your studio.
Then you can use this sheet to transfer your responses to the online underglaze research survey (all questions are the same).
This pdf can also be useful to keep track of your processes and results for your own records.
Click here to get it!
A Collaborative Research Project
Do you use commercial underglazes on your work? Have you had consistently successful results? Or have you experienced firing defects that you’ve been trying to solve?
To clarify, underglazes are different than glazes. They are colourful substances, generally made from a combination of clay, flux and pigment, that are painted onto clay to create decoration and designs. They are sometimes referred to as vitrified slips or engobes.
Underglazes generally stay where they are put, giving a “what you see is what you get” result that can be appealing when contrasted with the flow and unpredictability of glazes.
They come in a wide range of colours and are most often painted onto clay with a clear glaze applied over top to give a shiny, transparent surface that shows the underglaze design below.
Example of underglaze decoration with clear glaze overtop by Autumn Schweizer.
Over the past couple of years, I’ve seen lots of different firing issues related to using commercial underglaze from several different potters with multiple brands of underglaze.
I started to notice patterns between the issues. Since I don’t have personal experience with all the different brands of underglaze and wasn’t experiencing these issues myself, I wasn’t exactly sure how to solve these problems. But then every time someone presented another problem with their underglazes, I would think “That’s the same problem ‘so-and-so’ was having.”
I wanted to connect all the people who were experiencing these underglaze issues with each other so they could share these common problems and see if they could collectively figure out solutions.
I decided to make a post about it in my Facebook group – Understanding Glazes with Sue. After all, there are over 12,000 potters in the group. Sure enough, lots of people chimed in and shared their underglaze woes.
You can see the post here. (If you’re not in the Facebook group yet, you can request to join. Be sure to answer the membership questions so you’ll be approved.
The main underglaze issues:
The glaze on top of the underglaze crawled or pulled away, exposing underglazed clay underneath.
Photo: Priscilla Lentini
Both the glaze and the underglaze bubbled, crawled or pulled away, exposing bare clay underneath.
Photo: Peter Eastwood
Photo: Autumn Schweizer
Photo: Emily Price
Glaze pinholed, blistered or bubbled over top of the underglaze but not elsewhere.
Photo: Tanya Everard
Photo: Tanya Everard
Glaze seemed to be absorbed by the underglaze, leaving a rough glazed surface on underglazed sections but shiny and smooth elsewhere.
Photo: Tanya Everard
Photo: Barbara Krajewski
Glaze has “orange peel” texture over underglazed sections, but smooth elsewhere.
Photo: Evi Larsen
Shards of glaze and underglaze are flaking off. Glaze edges are sharp.
Photo: Staci Barron
Many factors to consider
When investigating firing issues, there are a lot of factors to consider that could contribute to the final result.
- What brand of underglaze was used?
- What type of clay was used?
- What stage was the clay at when the underglaze was applied?
- Did the underglaze go through a bisque firing before glazing?
- What type of glaze was used overtop?
- What temperature was the piece glaze fired to?
- What was the firing schedule?
The more info we know about the process leading up to a firing defect, the easier it will be to find answers and solutions.
It’s also equally important to understand the conditions that lead to successful results so we can compare.
Participate in a this “crowd sourced” underglaze research project
If you’ve ever used underglazes on your work, regardless of whether you’ve had problems with them or not, you’re invited to participate in this research project. I want to hear about the failures AND the successes.
One of the benefits of the internet is that we can all put our heads together and share our experiences to figure out solutions to these types of problems. The more data we can collect from people all over the world sharing their processes, the better chance we have at finding solutions.
Collective problem solving for the win!
In order to collect organized and concise data, I created a Google form that you can fill out to describe your underglaze experiences and all the factors that contributed to the final result.
Participate in the study by filling out this online survey
The survey will take approximately 10-15 minutes to complete.
Filling out this online survey will help my colleague Ronald Boersen and I put together possible theories for solving different types of issues with different brands of underglaze.
Once we have a theory, we’ll ask for volunteers to test the theories using the brands of underglazes they have available and see if we can collectively create a process for repeatable, successful results.
If you use commercial underglazes on your work, whether you’ve had issues with them or not, I’d love it if you would participate in this project. If you’ve had issues in the past and then figured out how to solve them, I would especially love to hear from you!
All participants who provide their email address will be kept informed of the progress of this project and any conclusions or solutions we have found. Results will also be published on my blog.
There’s an option on the form to add up to five images of your underglaze results. Google sign-in is required for file upload.
All data collected will remain anonymous with the exception of providing photo credit, with consent. Name and email collection are optional and will only be used for possible follow-up, clarification, and/or sharing of results. They will not be published or used for marketing purposes.
Thank you in advance for participating in this collaborative research project.
Zoom Chat about Underglaze issues
Watch our Zoom chat from Aug 20, 2021 about the survey results, a discussion about specific issues occurring with different types of underglazes, and some suggestions for testing to find solutions.
Ronald Boersen and I hosted a chat on Zoom where we went over some of the results we have so far in our underglaze research study and we chatted with a few folks about their personal experiences and troubles with underglazes.
During the call, we discussed why some colours might be causing issues while other aren’t, due to their different flux/refractory and/or off-gassing properties.
Another factor to consider is the possibly varied water content of the different bottles/jars of underglaze, due to natural evaporation from the time the underglaze is made to the time it is used.
Once, I ordered a bunch of underglazes and one of them was almost completely dried out when it arrived. I added water to reconstitute it, which ended up being too much water and it caused shrinkage/cracking issues with that particular colour of underglaze. When I removed the excess water off the top, it ended up working fine.
We also talked about how applying underglazes to different stages of clay dryness or bisque fired clay can lead to different application thicknesses, which could be contributing to different results.
Bisqueware is much more absorbent than leatherhard clay due to the water content when clay is leatherhard. You may get more vibrant results when applying underglaze to bisque, simply due to a thicker layer. But this thicker layer could also be affecting how the glaze applies to the bisque, leading to glaze adhesion issues.
An underglaze test you can try
If you want to do some experimenting to see if you can figure out what leads to issues with your underglazes, it’s important to keep most factors consistent, with very limited variables.
I designed a test that should give us fairly good information, if you want to try it out. It’s a test for 4 samples of clay in different stages. If it’s too much, you could pick one or two clay stages and try those out.
I’d love to see the results of this test, using just one of your problematic underglazes. The full test will take 2 bisque firings and one glaze firing to complete, so take your time.
- Choose one problematic underglaze and make sure it’s mixed really well before applying.
- Use the same paint brush for all samples.
- You’ll do one test on leatherhard clay, one on bone dry clay and 2 on bisque fired clay.
- Use one clay body and cut 4 slabs of clay that are all the same thickness. 6″x6″ should be large enough. Using a slab roller or a rolling pin with thickness guides will help for consistency.
- Once your clay samples have reached the leatherhard stage, take one sample and follow these steps to apply underglaze.
- Use your paint brush to apply 6 stripes of underglaze to your clay sample. Leave room to write the brand/colour of underglaze and other details on the sample. Re-dip your brush and stir underglaze before each stripe.
- Omit the first stripe and using the same method, apply a 2nd coat to the remaining 5 stripes.
- Omit the second stripe and using the same method, apply a 3rd coat to the remaining 4 stripes.
- Repeat this, skipping one stripe each time. This will result in 6 stripes of underglaze with 1-6 coats. Label each stripe with # of coats.
- Set sample aside to dry and repeat with one bone dry sample. Be sure to use the same underglaze and mix well.
- Once all 4 samples are bone dry and 2 are underglazed, put all 4 samples through a bisque firing (2 will be blank), all on the same shelf with witness cones next to them. Your bisque temp doesn’t matter, as long as you verify it with witness cones.
- Once your samples come out of the bisque, take a photo of your witness cones or label them and keep them for future reference.
- Use the same method to apply 6 stripes with 1-6 coats of the same underglaze to the 2 bisque fired samples (using the exact same paint brush).
- Put one of the 2 bisque samples through another bisque firing, with witness cones next to it. Photograph your witness cones after the firing.
- Once one of your bisque fired samples has been through a second bisque firing, you can glaze all 4 samples, using the same clear glaze and same application method for all. You can dip or brush the glaze, whichever you normally do.
- Before glazing, make note on the back of the tile, which direction the stripes are going. This is so you can apply 2 coats of glaze to half of the tile, ensuring all 6 stripes have one glaze coat on half and 2 glaze coats on the other half.
- Apply one coat of glaze to the whole tile, allow it to dry, and then apply another coat of glaze to half the tile, making sure that the second glaze coat covers half of all 6 stripes.
- Allow tiles to dry for at least a couple days to ensure they are completely dry before putting them into a glaze firing.
- Fire all tiles on the same shelf to your glaze temp, with witness cones next to them.
- Photograph your results and share them to my Facebook group – Understanding Glazes with Sue or email them to me directly.
I can’t wait to see your results!
Scroll back to the top to read about the underglaze issues people have been experiencing and our research project.
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!