Pottery/Making Miniature Clay Bricks
hello, I've read Your previous conversations with others and absolutely think that u can help with Your knowledge of the topic
lets start then, I had in mind a summer project of building a miniature medevil times castle using miniature building bricks. I made a choice of size of future bricks(05/0.5/1 inch) I bought AMACO X-15 clay and started to make those, I airdried it for few days, then put to kitchen oven seting temp for 180F to further eliminate any moisture. Today I prepared wood for my fire pit, threw apx500 little bricks, and almost cried when they started to pop like popcorn within 5 min in fire. now what kind of clay would You recommend to make those bricks( would like to have natural brick red/orange finish) and what tools or firing oven to get to make it right
Thank you for your recent question. I'm sorry for your loss on your project, but all is not lost. You have purchased a cone 5-10 high fire clay body that requires a kiln to fire to maturing temperature with a glaze. However, you can fire the clay to bisque fire (the natural color of the clay) to cone 05, which is a temperature of 1944 degrees F. Unfortunately, a pit fire will only reach temperatures of between 800 and 1200 degrees.
First, before you become discouraged, this can all be worked out, but testing is required. And in the future, when you want to do a project such as this, I suggest making tests of the project (just a few) and then you won't be doing a vast amount of work only to see the work destroyed as what happened in your message above.
The first thing you must do is calculate the exact size you want your finished bricks to be. You stated that you wanted them to be 05/.5/1 inch. I am going to assume you ment W .5" x H .5" x L 1.00". Making the bricks at this size will not result in the same size as you have to account for the remaining shrinkage due to chemical waters being expelled during firing and the clay particles compressing together. Therefore, we need to look at the shrinkage rate for the clay body. The data sheet states that the shrinkage rate is 12% when fired to maturity at cone 5. But since you will only need to fire to cone 05 for bisque to use the bricks, the shrinkage rate will be around 10%.
The reason I state around 10% is because you will have to test a few bricks to assure that this will be the shrinkage rate to do the calculations to obtain the correct size for your bricks due to the type of firing you will have available. Learning to calculate shrinkage rates takes some time to learn, so I will not go into that with you here, but will do so if the measurements do not come up correctly after your tests.
So, in order for your bricks to measure .5" x .5" x 1.00", you will need to make the bricks .55" x.55" x 1.10". When fired correctly, they should be the finished size you want. If not, we will need to calculate the shrinkage rate, so save the bricks you fire in the test and the information on the firing and we will recalculate the shrinkage later.
Next, create approximately 10 bricks. These will be your test bricks. Always create tests to work with so you don't ruin unnecessary amounts of clay.
If you have the ability to purchase a bisque bowl (a bowl that does not have any glaze on it), do so. You may be able to purchase one at a second hand store, or pottery store. The purpose for this bowl is to use it as a crucible to put the bricks into to prevent them from becoming exposed to direct heat from the pit.
Even though the bricks will dry and seem like all water has evaporated from them, there are still chemical waters within the clay particles that must surface during the heating of the clay. This occurs during the first 800 degrees of firing and is the most crucial part of the firing process. If the clay is exposed to the heat and fired too quickly, i.e. the heat can not be controlled to go up slowly like in a kiln, the water expels too fast, causing the clay to explode just as it did when you placed them directly into the pit fire. It would be best if you can find a bowl with a lid which is ideal for firing in a pit.
Finally, to fire the test pieces, build your fire in the pit by digging a hole at least 2 feet deep and 3 feet in circumference. Find a piece of tin, one that is used for roofing or siding a tin building and bend it to create a roof for the pit. It should be 1 foot above the ground at the point of the bend.
Use charcoal to start the fire. It will create a bed of coals that will stay hot and keep the fire going. A bag of charcoal will be needed. Place the crucible next to the bed of coals to warm as you are continuing to build the fire. Once the charcoal is burning, begin putting hard wood on the charcoal to create more coals.
Once you have a bed of coals that are at least one foot deep, place the crucible with the test bricks into the bed of coals. Make sure the crucible is sitting up and the lid is on and it is placed in the center of the pit. Place more wood around the crucible and continue to keep wood on the pit as it burns down for at least 4 hours. This is the time period needed to burn off the chemical waters and prevent the bricks from exploding.
Now, because a pit fire will only reach 1200 degrees at the peak of temperature and you need to reach 1944 degrees or higher, you will need to create a force to bring the temperature up. The easiest way to do this is to use a shop vacuum with the hose connected to the blower or exhaust. You will also need to provide a metal tube to attach to the plastic hose so that the plastic doesn't melt. My suggestion is to use a chain link fence pole, the 5 foot kind. Use duct tape to tape it to the end of the plastic hose. Lay the end of the pole along the pit, pointed at the crucible, approximately 6 inches away and approximately 1 inch below the bottom of the crucible. You may need to adjust this to create the following effect on the fire.
You want the fire to create a flame that blows around the crucible, not behind the crucible. It is important that the flame remains on the crucible and wraps around it, heating it and therefore, heating the inside. Remember, you are having to heat the outside hot enough to heat the inside to fire the maturing temperature. Once you have the correct position, leave the pole at that position during the entire firing.
Finally, continue to add wood as flames begin to decrease, but do not wait until they are no longer wrapping around the crucible as the temperature is no longer increasing. You want the temperature to continue to increase so that the crucible will begin to glow. This will take several hours. It may even take up to 6 hours, depending on the type of wood you use, the humidity outside and the proper set up of the pit. But what you are looking for is a crucible that is glowing as if it is on fire. When it reaches this point, fire for one more hour and then end the firing by turning off the blower.
You will want the crucible to slowly cool. This will take several hours. I recommend that you allow the pit to cool naturally for a day and then remove the crucible and let it cool for another several hours until you are able to remove the lid with your bare hands. Check your brick. The should be fired correctly without cracks.
The next step is to check the size of the bricks. Measure them and make sure they are of the size you require. If they are, then you are ready to create the remaining brick and fire again. However, if they are not of the correct size, contact me again and give me the dimensions and I will gladly adjust the creating size for you.
I hope this helps you create your bricks for your project. If you have further questions, please feel free to contact me again at any time. I am always at your service. Much luck to you and happy firing.
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat