Frequently Asked Questions
Topic: Mining indigenous clays:
I enjoy your web site and have found it very useful. I have started mining my own clay and have run into a problem and was hoping you might be willing to help. I got my clay into a slurry and screened it through a 35 mesh screen (it had very little sand or other impurities), let it dry until I could wedge it. Then I threw a couple of pots to see how it was going to handle. It worked very well. I set the pots aside to dry, as I would with purchased clay. The day before I planned to fire them they cracked and broke, very nice clean breaks all the way through the pots. One pot is in 4, almost equal, parts. Can you tell me what is going on here or guide me in a direction where I can get some help? I live in Wyoming and my only thought is that the clay has to high a concentration of bentonite. Thank you for your time and assistance. John
Thanks for the compliments about our site, Here is our answer to your questions:
We would suggest pulling some handles and mark two lines 10 cm. apart on each handle, after the clay is bone dry, measure the lines in millimeters. The number of millimeters less than 100 will give you your percent of shrinkage. If this is more than 3 to 5 percent, you have excessive shrinkage which can cause the cracking you are describing, regardless this still may be corrected by adding other ingredients to your clay body. If this is a stoneware clay body, add 10% potash feldspar, 15% flint, and 15% ball clay. If this is an earthenware clay body, add 8% ball clay and 2% iron oxide. If you suspect there is bentonite in your clay body, place a small amount of the dried clay powder in a jar about 1/4 full, add water to measure 1/2 full, stir really well and let settle over night. Bentonite will form a gel like substance which should be apparent on the surface of the settled clay by the next day. If this is the case, write us back and we will give you a different formula to help rectify your problem.
If you are creating a piece of pottery on the wheel and you discover there is a flaw in the clay that should you do? Should you go back to the wedging process and then start shaping the pot all over again? Are there any other flaws that might appear in the clay other than air bubbles? What should you do about them?
Thank you for your help. Frances
Yes there are other flaws that can occur when throwing clay on the wheel or during the hand building process. If it is an air bubble sometimes you can continue with your pull to the top of the pot and poke the bubble with your pin tool and push the air out of the bubble. If it is a large bubble and persists, you may want to start over with the piece. Other contaminants such as a scrap of sponge or other debris, should be dug out of the clay before you re wedge. You can check a lump of clay before wedging, especially scraps by running your wire through the clay in small intervals in two directions. The wire will generally catch on the debris and it can be pulled out. We would suggest that your wedging area be cleaned and free of any debris or dried chunks of clay. We would also suggest storing you clay in plastic bags in a cool moist area away from other containment's that could get into your clay.
Clay Type: Earthenware/stoneware
Firing Technique: Electric
Firing Range: Cone 5-6
Your Question: I have been throwing for a long time, but have trouble with tall cylinders. I can get to 8-10", but would like to know why clay buckles, breaks or refuses to go higher. Any help as to how I can throw to 15-17". Wonderful page you have. Thanks. Charles
Thanks for your nice comments about our web site, we really appreciate it. To answer the question clay buckles and breaks when throwing: When throwing your cylinder, your full height should be accomplished with 3 to 4 pulls. By the time you have completed your 4th pull enough time has elapsed that your clay begins to absorb excess water and will begin slumping down. Evenness in your walls from the bottom to the top is essential for throwing taller cylinders. For example: Take a two pound ball of clay throw a cylinder, remember a wider cylinder will be shorter than a narrower one. Get as much height as you can with 4 pulls. Take your potter's wire and cut your cylinder vertically, look at the wall of your pot, is there a lot of clay at the bottom? Does you wall narrow in the center and is there excess clay towards the top? If you have excess clay on the bottom you are not grabbing your clay from the bottom, if the wall is thin in the middle you are not pulling up evenly but pulling the clay from the bottom, you are squeezing together as you come upwards. These things can be corrected with practice and cutting your cylinders or pots in half vertically until you are satisfied that you are moving the clay the way that you want, then you will be ready to move on to larger cylinders. I find that weighing out my clay balls helps considerably because you begin to know the proportion of clay to the pieces and their size. Example: One pound clay = an 8 ounce mug, 2 pounds clay a large soup bowl, 3 pounds clay a 1 quart canister, 4 pounds clay a 2 quart pitcher. This proportion will help keep you from over throwing and trying to stretch a small amount of clay too far. Once you start throwing larger pieces your methods of pulling your clay is going to require more control and you may have to stand while throwing in order to keep your arms in the proper position as not to interfere with the walls of your pots using your fists instead of your fingers. I hope this helps you, if not, please feel free to contact us again or call to speak one on one.
Thanks and good luck,
Clay Type: Standard clay mines #266 Dark Brown Stoneware
Firing Technique: ^6 oxidation
Firing Range: ^6
I have been a potter for about 2.5 years and suddenly I am plagued by air bubbles when I throw more than 4 lbs. Of clay.
Do you have a technique to cure this? I have cut apart my wedged clay to check for air bubbles and I don't see any. This has been going on for about 2 months now.
Any help you can lend would be greatly appreciated.
Sounds like your plagued with incorporating air into your piece as you throw; as you use larger balls of clay this becomes easier to do.
Try this: Before you place your clay on the wheel make sure you have it well rounded like a ball, the reason for this as you slap it on the clay head, a flat surface can trap air under it. As you push your clay to center; be careful as you are pushing down and pulling that you don't have a large ridge coming down to the bottom, this will trap air in the clay. Also, maintaining your clay in a cone shape when pushing down with your thumbs, if you form a well in the center when you come back up and close it off can trap air inside. Always make sure to run your finger at your base and keep it clean and centered.
Test yourself: Before opening up your piece to throw; cut your clay in half in two directions (down and across) and see if you can find any air and where you are possibly having trouble. When establishing your bottom and leveling out, be careful that the ring of clay your are pulling isn't trapping air in the outside wall as you are pulling up.
To check this: Cut your pot in half and see where your air is.
Your Name: Lynda
Are there any clay products that pinch pots can be made from that do not require firing? We love pottery and would like to share the pinch pot experience with our five year old but do not have access to a kiln.
The Big Ceramics Store offers a crafting clay that you can bake in your oven. It is made specifically for little people to use, it is great for small clay projects. This particular clay is not like other fired clays in that it will not hold water. That is the only caution we have found with this product. Please find the link listed below to find out more information about the clay.
Your Name: Power Puff 13
Clay Type: Any
Where is clay from?
Clay is found around the world, it is the decomposition of stone, primarily granite, due to natural weathering and erosion, these deposits mix with organic materials and bacteria which form gel like substances within the clay and these organic make the clay plastic or sticky, this gives the clay the ability to be plastic and hold shape. During the firing process the organic compounds are burnt out and the heat melts the stone back together. Low fire leaves the clay porous but as you add higher heats they melt together and this is called vitrification. A good source for research on various clay bodies can be found on Encarta.
Hi Which type of clay would I need to use to make an imprint of my hand that wont stick to my hand.
A white earthenware for low fire an 06 clay body range. A white stoneware would work nice for high fire up to cone 10. You can stain the print with stains or glazes that match your firing range.
Your Name: Barbara
Firing Range: Cone 6
I am taking a beginner course and need a known clay body formula and a new clay body. Also a known glaze formula and a new glaze formula. Could you help me. I don't know where to start, Thanks Barbara
We have some cone 6 clay recipes on our web site that would be known formulas, you are more than welcome to copy and use them. When you say a unknown formula, do you mean you need to formulate a new glaze and clay recipe? Are you an art student? If you are a student we can recommend some references for you that will help you in formulating both clay and glaze
recipes. One other question! Are you needing to formulate a clay recipe from indigenous clay?
Here is a known cone 6 glaze recipe:
30% Gerstley Borate
15% Ball Clay
Thanks so much for your help. I have found a Stoneware Clay Body Recipe which is Cone 10 and I would like to use; however, it calls for one item which I do not have can you tell me a substitute for: H.C. Spinks Foundry Hill Creme or what that might be???? Thanks again for your help. Barbara
The entire recipe was:
Custer Feldspar 12.3
6 Tile Clay 09.5
Cedar Heights Goldart 19.8
Cedar Heights Redart 03.5
Hawthorne Bond Fireclay 07.9
Kentucky Ball Clay (OM 4) 19.8
H.C. Spinks Foundry Hill Creme 19.8 ???
Fine Grog 01.8
This was from a Ceramics Monthly December 19955 article.
Foundry Hill Creme is an air dried ball clay. I found a couple of places that you can purchase it from or you can adjust your recipe. To adjust the recipe divide the 19.8% by 2 and add to your amounts of the Hawthorne Bond and Kentucky Ball clay. The first link provided below is to the Big Ceramic Store on line and they have the Spinks Foundry Creme , the next link is to L & R Specialties here in Missouri
This was a great help and I really appreciate your help. Thanks so much.
Your Name: Jane
Clay Type: Stoneware 48-M
Firing Technique: Elec. kiln
Firing Range: Cone 05
What percentage will shrinkage be for this clay, green ware and bisque firing???
Make a bar of clay long enough to put two marks 10 centimeters apart, such as an extruded handle. Let this dry to bone dry consistency, measure the marks again, the reduction in millimeters for example: If 5 millimeters shorter between marks means that your clay has shrunk 5% to the bone dry stage. Now bisque: Measure the marks again, the reduction in millimeters represents the percent of shrinkage at the bisque stage. Now final firing shrinkage: Fire at normal firing, check your marks again, you should see the total amount of shrinkage from wet clay to firing. As a general rule, most clay will shrink approximately 12 to 15 percent on the finished piece. When using commercial clays, shrinkage rates are available from the distributor and manufacturer.
Topic: Glazes and slips, burnishing
Your Name: Kay
Clay Type: Stoneware
Firing Technique: Oxidation
Firing Range: Cone 6
Hi - I'm learning how to make my own glazes - and the recipes I've been given uses percentages as measure instead of grams.... Not sure how to proceed. Can you explain this to me? Thanks...
Hi Kay, Mixing glass is very exciting and rewarding. Percentage composition is a proportional measure . 100 units of weight. E.g. pounds,ounces,grams. I use pounds or kilograms for large glaze buckets and ounces or grams for testing. Lets say you have this cone 6 glaze, (9.3% zinc oxide.25.9%nepheline syenite,8.7%lithium carbonate 11.3%kaolin,9.7calcined kaolin and 35.1%flint. add these numbers = 100. You decide what unit. I use pounds but I don't need a 100 pounds of glaze I need 10. So divide everything by 10 or move the decimal a notch to the left. Now the recipe might require a % of an oxide or stain for color remember to devoid that as well this is an addition to the base glaze and can be changed to make different color glazes. Well I hope this clarifies your problem, have fun. Ken George
Your Name: Victoria
Firing Technique: Oxidation
Firing Range: 1786
Glazes: Colored slip
Hello wonderful web site. I have been experimenting with burnishing. my pots still fire dull,no luster can you give me some help? thank you
It would be helpful to know what clay body you are using. Some clay bodies are more suited for burnishing than others, if it is too high a firing body, the clay body will open up too much at bisque temps causing the surface to dull. You may want to try some low firing earthenware's. Some good burnishing clay bodies are available from Mile Hi Ceramics in Denver, their web address is http://WWW . In general burnished ware is fired in reduction. The reduction process results in a much richer burnished finish. We would suggest reading about other firing techniques suitable for burnished pots, such as, dung firings and those described by potter Michael Wiesner in an issue of Ceramics Monthly magazine. Studying about poly chrome slips will be helpful in learning more about luster in burnished ware. A great source of information is Ceramics Monthly magazine, on line: http://www.ceramicsmonthly.org
How do I mix slip and clay to form glaze colors;
You are correct in how to mix the slip using your clay and glaze. You will want a consistency that is not too, thick but paintable, more like a latex paint. Using a 80 mesh screen to strain you slip will also help to create a finer batter in eliminating some of the coarseness and grog. For your colors: You can create a wide pallet of colors by mixing colored stains or pigments and by using a clear base glaze with your slip. Using your basic slip mixture you can create a wider pallet by testing each stain and adding more or less of the specific color each time. Make sure you are using a stain that is made for your firing ranges. If you would like your slip areas left exposed or unglazed, you can place wax resist over those areas before applying your last coat of glaze. As far as firing results, you can only be sure after the firing. Predictable results comes with testing and more testing. Take into account if you are using an electric kiln a gas kiln, wood or Raku you results will differ with each method, also oxidation and reduction will affect your colors and final results as well. The type of clear overglaze you are using can also affect the final outcome of the glazes and stains.
Clay Type: stone ware and porcelain, high fire
Firing Technique: Electric
Firing Range: Uncertain
I have been studying pottery for a while now and someone has offered to give me a wheel and an electric kiln. I prefer to do high fire work and am looking for a list of food safe high fire glazes, probably ranging from cone 8 to cone 11 or 12. The art center where I study does the glaze mixing and firing so these are skills I''m going to have to teach myself via books, asking others, and experimentation. Can you recommend a book, site, etc. that has a list of glaze recipes that are clearly noted whether they are food safe or no? Any help is much appreciated. I'm disabled and have a hope of perhaps supplementing my income this way. Thank you
One of the best sources available for glaze recipes and updates on glazes and the study of pottery in general is Ceramics Monthly magazine. They offer a wealth of information on glazes and which ones are toxic and non toxic. Here is the link to their on line site: http://www.ceramicsmonthly.org/ You may also want to visit http://www.amazon.com/ for reference books on glazes and firing. It is important to study how the clay body, glazes and firing all work and react to each other to produce the finished piece you want. As a general rule of thumb: High firing glazes are usually non toxic.
I've been looking for a way to add texture and color to the outside of my pieces. I read the clay recipe section and saw the one on colored slip. Do I mix the clay I'm using to throw the pot with water and underglaze? ( I'm using a low-fire clay) If so, does the pot need to be rather wet still when I add this slip? If I make the slip rather thick will it leave a texture after the bisque fire?
Lets start with your grog problem, you probably need a 80 mesh sieve, but you can try this, take a one quart mason jar and fill it 1/2 full of your clay and fill it the rest of the way up with water, place a lid on it and shake it until all of the clay is suspended, then set it aside and let it settle, you will find that the finest clay will come to the top, remove the very top layer using a spoon, this clay should be free of grog. You should be able to see bands of clay through the glass. If you don't want to purchase a sieve you can go to the Goodwill or somewhere along those lines and look for a finer mesh screen on any household item that has a screen, such as a grease splatter screen or a flour sifter or duster, they may even have these new at reasonable prices at Wal Mart or K Mart.
On to the glaze/clay antique colors. It sounds like the appearance you are trying to achieve is done by adding ash to your glaze or a combination of an ash glaze with a different firing method such as a pit or raku.
Glaze Recipe question:
Clay Type: High fire
Firing Technique: Electric
Firing Range: Cone 7-10
Glazes: My own rec.
I am having a problem with a pink glaze. Have not tested it. Do not know if it will be the color I want. Should turn out pink, if I am wrong might turn out peach, I am not sure of my chrome amount. Tried adding extra Bentonite for cohesiveness, but after application, it cracks when dry. My question is what causes a glaze to crack? What ingredient can I add? I thought about just firing a piece with the cracked glaze but it sort of separates and peels off as well. What can I add to give my pink glaze a gold hue, if it I can ever get it to work?
Just for our reference, what type of clay body are you using? Stoneware, Porcelain, etc...
There can be a couple of reasons why your glaze is cracking: 1) Is you glaze too thick? You may need to add more water 2) Bentonite is added for uniform suspension of your glaze materials. Some glazes will settle heavy to the bottom and be hard to mix, certain materials won't stay suspended. Bentonite shrinks a great deal when it is dry, try to keep your recipe to include no more than .50% Bentonite. For cohesion and reduction of cracking add V-gum, start with about .50%, if small cracks still appear you may be able to rub them out, once dry. If you are still having problems with crawling some ingredients will have to be subsisted to assure a proper glaze/clay body fit. I would really need to see a copy of your glaze recipe to suggest the amount of chrome to add to your glaze or to suggest any changes to the recipe to reach your desired color. Please send a copy of the recipe and include the clay type you are using and the type of chrome you are adding.
Topic: Clay Decoration
Clay Type: White clay
Firing Technique: Pit firing
Firing Range: Outside
How would you create different effects on the clay?
You might try burnishing or burnishing with paint oxides, hand carvings or hand alterations.
Topic: Pottery Cracks
Your Name: Mudworks Pottery
Clay Type: Cone 5 Laguna red calico
Firing Technique: Oxidation cone 5
Firing Range: 2165 degrees
Glazes: Laguna cone 5
An 8" plate with trimmed foot,bottom a little thinner than sides, glazed Ok, not too thick, cracked completely apart about 3 hrs after taken out of the kiln. No sign of any cracks after bisque fire, even after out of glaze load. What do you think caused this to happen?
Lets get right to the problem, cracks on cooling can have do with differences of the glaze to the clay body. Cracks can also occur if there was a contamination in the clay such as plaster or lime or other foreign material that after firing expanded within the vitrified body. Foreign matter can form extreme amounts of pressure and can sometimes crack the piece during firing or I have seen this happen days or even month later. You can crack the piece further and identify the point of the crack and determine if there is in fact something in the clay body. Other possible causes: Over vitrification during firing, or under bisquing during the bisque firing. Under bisque can leave excess amounts of organics that can be trapped in the clay body during the final firing. Hopefully this is only a one piece occurrence. I saw a lime contamination in an entire batch of clay that took out an entire load of pottery (about 500) pieces in a 6 month time.
Topic: Hand Building
Hello, I am working on a badge for Girl Scouts and we have to make a coil pot out of clay. Can you please explain what a coil pot is? Thank you very much for your help.
A coil pot is simply a method of hand building in which the clay artist rolls the clay into long rope like stings and attaches them together with clay slip to form art pieces. Coil pots are very ancient and were used for water vessels in ancient times. When making a coil pot make sure your snakes are of consistent thickness. You can find a recipe for clay slip on our clay recipe page.