You are here:

Pottery/Gloss Ceramics Firing Results in Matte Finish

Advertisement


Question
QUESTION: Actually this is a 2 part question about the same subject:
1) I fired my ceramic sculpture to 04 because I have had great success getting brighter colors. I do this all the time using the same clay, underglazes, and clear glaze top coat. However, this time my piece came out looking matte and cloudy in places and the colors seem dull or flat. I have no idea why, do you?
2) Because my ceramic sculpture came out looking dull I want to refire it in hopes I will get a better result. I work by painting underglazes on my pieces and top them with a clear shiny or satin finish and then fire them to 04. Can I paint underglaze on top of a finished piece that has a shiny fired finish already on it and then refire it and have new layers of underglaze and clear glaze stay in place?  Sanding the surface is not an option for me because of the angles, etc.

ANSWER: Dear MaryAnn,

Thank you for your recent questions. First, let's tackle question one. The kiln may have reached cone maturing temperature, but didn't stay there long enough to soak the glaze materials, which prevented the flux ingredients in the glaze from fusing with the refractory ingredients. This leads to the second question, can you re-fire. Yes, you can.

However, you will not need to re-glaze your pieces in any way as it is not the lack of glaze that has caused the problem, but rather the length of time the glaze was in the kiln.

My suggestion to you is to re-fire the pieces, but when the kiln reaches cone 04, prevent the kiln from turning off and allow the sculptures to soak for at least 30 minutes. This should give the glaze enough time to flux, therefore, melting the glaze as it should and causing the flux ingredients to meld with the refractory ingredients.

As a note to you, your elements in your kiln may be weakening and may not be reaching cone. You may want to check them by doing a dry cone firing to assure your kiln is reaching cone temperature. Do this by placing a cone in the kiln and firing to cone, watching the cone to assure it bends at the appropriate temperature. If you use a kiln sitter, you may want to check it too. It may be that it is out of calibration and needs to be adjusted. Test it by placing a cone one cone higher than the maturing cone in the kiln to assure it is working properly.

You didn't mention your kiln model. If you have the kiln manual, it should explain how to check your elements and your kiln sitter. If you do not have the manual and want a copy of it, let me know your kiln model and name and I will gladly supply you with the manual.

I hope this answers your questions. Please let me know if I can be of further service to you. I am always at your service. Much luck to you.

Sincerely,
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat
www.earthstokenfire.com

---------- FOLLOW-UP ----------

QUESTION: First I want to say, Thank you for your previous answer. I did what you suggested: I refired the piece without reglazing it. I set the hold time to 30 minutes.   It came out bright, shiny abd clear. Perfect!

This summer I have been having a problem with the finished glaze chipping, really just falling off, my finished pieces which are curved platters that are made up of 50+ pieces. I have nade these in the past with no problems. Now, however, when I  open the kiln I can already see pieces of glaze that have fallen off. Just little pieces or silvers. At first I thought it was the new clay - I had switched to paper clay. So I changed back to using either B mix or 04 low fire clay. I paint the underglaze on my piece when it's greenware. I just open my bisque kiln and I see olaces where the underglaze has peeled back from the clay.  All I can think of is maybe my clay needs to be wetter when I paint the underglaze on. Please help!  Thank you in advance!

ANSWER: Dear MaryAnn,

Thank you for your recent question. Chipping or peeling of a glaze is due more to glaze fit than any other problem. But, in order to properly answer your question, I will need to know exactly what clay you are using and what glaze you are using, including the maturing cones for each.

Once I receive this information, I will be able to answer your question with preciseness. I look forward to your reply.

Sincerely,
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat
www.earthstokenfire.com

---------- FOLLOW-UP ----------

QUESTION: Up was using 04 paper clay:
1. painting my greenware with Mayco underglaze,
2. then bisquing it at 04
3. Then applying a clear glaze either
       A.Mayco crystal clear brushing which fires to 05-06
       B. Duncan satin glaze which fires to 06
       C. Duncan pure brilliance which fires to 06.
   I'm just now noticing these cones. I had fired with all these clear glazes to 04. Could this be the problem?

I did just finished a platter using B mix, bisquing at cone 6 and finishing with (C) Duncan clear glaze with great success.

I had switched to low fire glAze to save on electricity and wear on my kiln ... then I switched to low fire paper clay to made my platters lighter. I like e I. Las Vegas and work in my garage so during all my problems the room temperature was at least 100 degrees, in case this matters. The greenware was therefore a lot drier when I painted my underglaze on it and the pieces dried a lot faster.

I hope I have supplied you with enough information. Thank you in advance for your help.

Answer
Dear MaryAnn,

Thank you for supplying the answers to the questions I asked about the type of clay and glazes you are using.

First, let me say that in the future, when you are in high temperatures, trying to slow dry green ware, place a cup of water next to the ware and then cover both with a plastic bag. This will create a humidified bubble that will slow the drying phase down. This will prevent cracking, even minute cracks that you can't see.

Now on to your glaze problem. Since you have disclosed you are making functional ware, it is very important that your ware is glazed and fired so that it is food safe. Glazes that do not "fit" a clay body will cause chipping, peeling and cracks, even those you can not see. This causes ware to become a harbor for bacteria.

First, let's talk about Cones. This applies to both clay and glazes and maturing temperatures for both. The final firing is the highest fire you should accomplish when you are finishing your ware. So the highest firing will be the glaze firing, which sets the piece, fusing the clay and glaze together in a thin undercoating where they both bond together. This fusion is very important as it prevents the glaze from separating from the clay and makes the piece strong and safe to use with food. With this said, let's move to Cone types.

Cones range from 022 to 14 in the pottery field. To help you remember what is the coolest and what is the hottest, just remember Cones with a zero are like negative numbers, while those without a zero are positive. Therefore, Cone 1 though 14 are hotter Cones than Cones 01 through 022. Also, just as the zero Cones are cooler than the non-zero Cones, the numbering system represents a cooler or hotter degree in temperature. For instance, Cone 1 is cooler than Cone 14. Cone 5 is cooler than Cone 10 and so on. On the opposite end, Cone 02 is hotter than Cone 05 and Cone 04 is hotter than Cone 010.

Unfortunately, many potters confuse the Cones, especially when working with Cone 04, Cone 05, Cone 06 and Cone 5. But what is most important is to understand that you can not complete a maturing glaze firing successfully using a glaze that has a Cone maturing temperature hotter than the clay Cone maturing temperature.

Let me explain. Let's say you use a Cone 04 clay body, such as your paper clay. The reason it is called Cone 04 is because the maturing temperature, or the temperature in which the clay body is fully fired is Cone 04. If you were to fire any higher, the clay would distort and even melt in the kiln. This also means that you can not use a glaze that matures any greater than a Cone 04. Using a Cone 03 or higher would cause the clay to distort or melt and you would have a pile of melted raw materials on the bottom of your kiln. What a mess!

Now, that said, I must add that you can, with success, often fire one or two cones hotter, however, you need to test this on individual test tiles to make sure the clay will withstand the hotter Cone. This is said, just in case you choose to use a higher maturing temperature glaze.

When firing a clay body, it has always been my practice to fire just under the maturing Cone. I do this to allow the pores of the clay body to remain open to accept the glaze better. Being that I am a chemist and test and design clay and glazes, I have found that this simple step allows a better fusion with the clay and glaze during the glaze firing.

But since you are underglazing your paper clay, and then using a overglaze post firing, you will want to mature your underglaze and paper clay by firing it to Cone 04, even testing it to Cone 03.

You stated that you noticed the overglazes as being Cone 05-06 range and that you had fired them to Cone 04. This could have been the problem, since the glaze was fired higher than the maturing temperature, which would have caused a shrinkage against the skin of the clay body, causing peeling and chipping. I would try again on some test tiles.

Finally, on your platter that you had success with, unfortunately, as described above, your piece is not properly fused. It is not just what you see in beauty from the glaze itself and the form of the platter that you should be concerned with. As a producer of functional ware, you must be concerned with the safety of ware. You stated that you are using B Mix, however, you didn't state whether it is Cone 5-6 or Cone 10. I am going to assume it is Cone 5-6 because you bisqued at Cone 6.

To create successful functional ware that is safe and has matured correctly, this is my suggestion to you that will also save you time and firing costs:

First, you will need to switch your glazes to mid-range if you are going to use the B Mix Clay. The current Cone 05-06 glazes are not fusing to the clay body and will eventually chip away. Spectrum makes some wonderful mid-range glazes that you can look at on Bailey Ceramic Supply Website at https://www.baileypottery.com/glazes/spectrum1100.htm  The nice thing about these glazes is that they can be purchased as gloss or satin so there is no need to apply a clear overglaze, saving you the extra firing.

Create your ware and bisque fire to Cone 05.

Apply your glaze, allow to dry.

Fire the piece to Cone 6.

If you choose to stay with the paper clay, check the firing you did with the clear glazes. However, there are also steps you can do to eliminate so much firing. If you are interested in learning this, please feel free to contact me again. You may have to contact me in a new question, as AllExperts limits the number of replies volunteers can give, but do not hesitate to ask any questions you may have. I am always at your service. Hope this has given you the information you need to create successful pottery ware.

Good luck and let me know how things turn out!

Sincerely,
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat
www.earthstokenfire.com

Pottery

All Answers


Answers by Expert:


Ask Experts

Volunteer


Ti Phillips

Expertise

Will answer any questions on hand building, wheel, glaze, firing. Speciality questions to include those in glaze calculation and development, firing techniques. Please do not send questions on identifying pottery. Although I would love to add this to my question topics, I have a retreat to run as well as the studio and volunteering on AllExperts, and therefore do not have the available time to research indentification and marks. Thank you for understanding.

Experience

Experience includes 30 years in pottery design and education. Have taught online and studio classes worldwide for the last 20 years. Own a pottery retreat specializing in firing techniques. Have 12 years solid experience in glaze calculation and formulation as well as problem solving in glaze chemistry. I am the first potter in the United States to have developed a complete package of pottery equipment blueprints for a studio. The blueprints include wheels, kilns, studio furniture, wedgeboards, raku kilns, slab rollers, ball mills and studio tools.

Organizations
Alliance of Pottery Artists Worldwide Association

Publications
Ceramic Industry - PPP Wyndstryder Press - Pottery Journal

Education/Credentials
University of Sciences and Art's of Oklahoma, studied under Professor Jaymes Dudding.

Awards and Honors
Potter of the year with APAWA, various awards for showmanship and design.

Past/Present Clients
Available upon request.

©2016 About.com. All rights reserved.