Firing, Cones and Clay

Transcription

1 Firing, Cones and Clay Firing Process What Is Firing? Bisque Firing Firing is the process of bringing clay and glazes (the applied surface) up to a high temperature. The final aim is to heat the object to the point that the clay and glazes are mature that is, that they have reached their optimal level of melting. (Pots and other clay objects won t look melted; their melting is on the molecular level.) Clay cannot be used without firing it. If clay does not get fired it will remain weak and break apart or disintegrate. Typically it takes two steps to fire clay, a bisque firing and a glaze firing. However, some ceramist may fire their objects 6 times or more. Bisque firing is the first time the pots go through high temperature heating. It is done in order to vitrify the clay pots enough that they won t be harmed when glazes are applied, but not vitrified to such an extent that the glaze won t adhere correctly. The bone-dry greenware is very fragile and must be loaded into the kiln with a great deal of care. The kiln is closed and heating is slowly begun. A very slow temperature rise is critical. During the beginning of the bisque firing, the last of the atmospheric water is driven out of the clay. If heated too quickly, the water will turn into steam while inside the clay body, causing the clay to burst. Once the ware is bisque it is ready to glaze. Types of Firing Glaze Firing Kiln Raku Low-fire Oxidation High-fire A glaze gives clay its color and finish. Once glazes have been applied to the bisqueware and have had a chance to dry, the ware is carefully loaded into the kiln for the glaze firing. Pots cannot be allowed to touch at all, or the glazes will melt together, welding the pots together. There also cannot be ANY glaze on the bottom or it will stick to the shelf. The kiln is heated slowly to the proper temperature to bring the clay and glazes to maturity, and then slowly cooled again. Only after the kiln has cooled will it be opened, unloaded and ready to use. A kiln is used to fire ceramic objects. A kiln is a furnace or type of oven for burning, baking, or drying. In ceramics there are many types of kilns that heat in different ways. Typically the heat source is electric, gas or wood. There are many types of firing with different processes. Firing gets very complicated and technical. This is a brief breakdown of the firings available to you. Raku is a very low-temperature firing resulting in pourous ware that is not appropriate for eating and drinking. Raku is a very quick firing that lasts from 45 minutes to an hour. Once the ware reaches temperature (around cone 06) the kiln is opened and the ware is removed while glowing hot. The ware is then put into a can of combustible materials such as paper or sawdust. The ware cools for 45 minutes then viola! You will do this type of firing!!! Oxidation firing is typically done in an electric kiln that uses electricity to heat up. Oxygen is free to interact with the glazes when firing. Oxidation firing allows very bright, rich colors and takes about 16 hours from load in to load out. Reduction firing is typically done in a gas or other fuel-burning kiln. In reduction

2 Reduction firing, oxygen is prevented from interacting with the glazes during glaze maturation. This is done by adjusting the fuel and air in a gas kiln to reduce the available oxygen. Reduction firing typically is used to obtain mottled, rich, earthy colors, often the iron from the clay shows through and gives a speckled look. Typically high temperatures are used in reduction firings and it takes 4 days from load in to load out. Pyrometric Cones Purpose of cones The cones, often used in sets of three to sets of 8 are positioned in a kiln with the wares to be fired and provide a visual indication of when the wares have reached a required state of maturity. A cones maturity is a combination of TIME and TEMPERATURE. Thus, pyrometric cones give a temperature equivalent, they are not simply measuring temperature. In other words, pyrometric cones measure time and temperature. For example: If a kiln is being fired slow cone 04 may drop (or bend) at 1935 degrees. If a kiln is being fired fast cone 04 may drop at 1945 degrees. Cone Shape Bending of Cones The pyrometric cone is described by Dodd and Murfin (1994) as 'A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base at a definitive temperature. The bend determines if the kiln was under fired, over-fired or properly fired Cone Pack When the cones are placed in a group and ready to fire it is called a cone pack. The reson for the number of cones is to determine at what state the kiln is at. Once the guide cone goes down the kiln operator should keep a close eye on the kiln. This cone pack has not been fired. It is used for a high-fire reduction kiln. Notice all those cones!!! Every one of them is there for a reason and lets the kiln operator know what is going on inside the kiln. They can then adjust the air, gas and flu accordingly. Pyrometric Cone Scale Pyrometric cones range from cone There is a cone for every number between these two numbers. Notice that the scale starts with a zero in front of the number. The cone scale runs like a negative to positive scale and goes from cool to hot.

3 Standard Cones/Temperature COOLEST HOTTEST If you see a number with a zero in front you would not say zero 4, you would say Oh 4. Temp F Firing/clay type 270/hr Lusters, enamels, over-glaze, china paint usually firing b/n cone 020 and 016 Any clay Bisque usually firing b/n cone 08 and 05 Any clay Low-fire glaze usually firing b/n cone 05 and 03 up to cone 1 Earthenware clay but high-fire can be fired low Mid range glaze usually firing b/n cone 3 and 6 Mid range stoneware or high-fire-stoneware, porcelain High fire glaze usually firing b/n cone 8 and 12 High-fire stoneware, porcelain WHAT IS CLAY Clay Clay Body Clay is rock that has been broken down by the natural elements of erosion. It is broken down into tiny particles. These clay particles are made up of alumina, silica and chemically combined water and are often found as dry powders in the earth. Clay deposits are found feet deep in the earths crust A unique quality of clay is its plasticity. The smaller the particle size the greater the plasticity of the natural clay type. The structure of a clay particle is flat, shingle-like, and hexagonal. The particles are positive and negatively charged and slide to give support to one another. Clay on its own, is not what we work with. The plastic clay that we make objects out of is considered a clay body. A clay body is chosen for desired qualities. One clay body can be very different from another. Clay body consists of Clay, Filler, Flux. Usually the different dry powders are mixed in a clay mixer or pug mill but sometimes they are found together in the earth thus creating a clay that can be dug from the earth in the plastic state and used to make objects. Clay Flux Filler Chosen for its color and plasticity and hardening temperature. (body) Lowers the hardening temperature of clay. It is the binder that melts the particles together (glue) Makes the clay stronger by decreasing the shrinkage of the clay Fillers that burn away: sawdust, straw, nylon fibers Fillers that stay in the clay: grog (fired & ground up clay), sand, fiberglass (bones)

4 3 MAIN CLASSIFICATIONS There are many more types of clay than this, but, here are the main 3 Earthenware Stoneware Porcelain This is a low fire clay body generally firing between cone Very porous / open clay weaker than high- fire at maturity. Must be glazed to prevent water leaking Terra cotta fits in this category Usually red but can be white Iron oxide is the source of red in red earthenware. Iron has a very low melting point and is rarely found in high- fire materials. Stains This is a mid- range to high- fire clay body generally firing between cone 5-10 Very sturdy clay that becomes waterproof at its maturing temperature. Do not need to glaze. Usually buff or grey in color once fired Easy to use because it has fillers good for throwing and sculpture This is a high- fire clay body generally firing between cone 8-12 Extremely small particles so it shrinks and cracks very easily because there are little to no fillers White in color due to its purity Once fired to its maturity it becomes glass- like and waterproof Finicky and hard to work with but the results of porcelain are unmatched by any other clay body STAGES OF CLAY BODIES Greenware Slip ALL unfired clay is considered greenware or often called green. This term is used often. So if your work has not been in a kiln, it is considered green. liquid clay Used as a glue to attach wet and leather hard clay together Can be colored and applied to surface of clay. Slip is a clay therefore it will not stick to kiln shelves or to another clay object in the firing Engobes and underglazes also fit in the slip category More information on slip can be found in the ALL ABOUT SLIPS AND GLAZES material sheet. Wet Leather-hard Bone dry soft and plastic easy to form. This is what we build with in the soft slab, coil, pinch and wheel- thrown methods of building. (greenware) Stiff but still workable. Same color as wet clay. Great for slab building and surface decoration. (greenware) Dry, no longer cold to touch. Color changes to lighter shade than when wet. No longer workable. Most fragile state. Ready to fire (greenware). It is at this state that it is safe to put clay in a bisque kiln. Bisque Clay has been fired once at a low temp. Usually between cone 010 and 04 Bisque releases all the water and brings the object to a state that glaze can be applied no longer dissolves in water Vitreous or Clay has been fired to its maturing temperature. The physical and chemical

5 Vitrified water has been fired off and it is at its strongest state. Usually glaze has been applied at the bisque state and the object is fired to a higher temperature melting the glaze and bringing the clay body to its maturity thus vitrified. Slip and Score Slip and score is the method used to glue two pieces of clay together. In order to successfully adhere clay you must scratch the surface (score) and apply slip in the grooves. Two pieces of clay WILL NOT stick together if you do not do this (unless they re wet.) They may look like they are stuck but they will come apart at bone dry or bisque. There are a few things to know about when to slip and score. Different stages of clay require different methods of slipping and scoring. Wet to wet Wet to leather Leather to leather There is no need to slip and score if you are putting two pieces of wet clay together. You just need to be sure to really smoosh and compress that area. You must slip and score when putting a wet piece of clay to a leather hard surface. Be sure to slip and score BOTH surfaces. When adhering two leather hard pieces of clay you must add a coil or disc of WET CLAY between the two pieces. You want to slip and score both leather hard surfaces, apply the wet coil or disc to one of the leather hard surfaces and slip and score the wet coil or disc. Attach the remaining leather hard piece. In other words, wet clay must reside between two pieces of leather hard clay. PLASTICITY AND SHRINKAGE Plasticity Plasticity is the ability to easily shape clay. Clay with smaller particles such as porcelain, are easier to shape whereas larger particle clays are harder to shape. Laws of plasticity If a clay has small particles more plasticity more shrinkage more cracking If a clay has large particles less plasticity less shrinkage less cracking Shrinkage Clay shrinks as the water evaporates from it or water is removed by heat. The heat of the kiln also draws the clay particles closer together. The smaller the particles of clay (the more plastic) the higher the shrinkage. Clay shrinks up to 14% Clays with large particles and fillers such as grog shrink less Most shrinkage occurs between wet and leather- hard stage (5%- 7%) and bisque to vitrified (5%- 7%)