How much does clay composition effect shrinkage? Reinforce your students learning with this exercise!
In this exercise, your students will create cylinders from different types of clay and compare both qualitatively and quantitatively physical aspects of each. Students will take an experimental approach, creating pieces which are purpose built to facilitate scientific observations, while controlling for multiple variables.
Learning Objectives
Students will be able to differentiate the clay bodies according to their physical properties. Students can describe how these physical properties are the result of differences in the molecular makeups of the clay. Students observe the varying shrinkage rates of the clays involved, and are able to explain how the molecular composition of the clay determines this.
By interacting with this lesson using an experimental approach, students will reinforce their scientific understanding of the chemistry involved in clay composition by physically interacting with what are otherwise abstract phenomena.
Required Materials
A full studio is recommended for this exercise. Specific equipment required include:
2lbs of clay of at least two bodies (i.e. stoneware AND porcelain) per student
Pottery wheels
Bisque firing kiln (gas or electric, must fire up to 2552°F)
Storage space for drying cylinders
Directions
Step 1 - Throw
Students will begin by wedging three .5lbs balls of clay of each type being used in the lesson; possible clay types including earthenware, stoneware, and porcelain. Have participating students throw three cylinders (3.5"x3") from each clay type to be compared. Use of a throwing guide is recommended to create consistency across pieces. All cylinders should have exactly the same starting dimensions, as later observations depend on the starting point being constant.
Measure: Students should create a table and mark the initial weight of clay used for each of the three 'replicates' they will be producing. After throwing a piece, students will record the initial height and width of each cylinder thrown next to it's initial weight.
Key Observations: Students should familiarize themselves while they're centering with the material properties of the clay. These include rigidity, graininess, dryness, and plasticity. The clay bodies used will feel very different to work with; let students draw their own conclusions of the underlying properties.
Constraints: Students unfamiliar with wheel throwing may struggle to produce a cylinder, let alone three consistently shaped ones. If still participating in the lesson, students can instead use strips of clay measured to exact dimensions.
Step 2 - Dry
Allow your students to store their pieces, being sure they record which exact piece corresponds to the measurements taken previously. When the pieces are bone dry, approximately one week later, have students return to these cylinders and remeasure them. After drying, they will have shrank by varying degrees depending on the clay body.
Measure: Students should record the new height and width of the corresponding pieces, and calculate individual shrinkage rates. Shrinkage rates are calculated as: ( Thrown Length - Dry Length ) / Thrown Length. Have students then average these rates by clay body, marking their works and result.
Key Observations: Students should observe consistent shrinkage across cylinders with the same clay body, but varying across other bodies. Replicates allow for individual variability of these traits to be controlled, and will give exposure to basic statistical methods and experimental design.
Step 3 - Fire
Fire all cylinders made by students up to cone 6, with a normal bisque firing schedule. Ensure students have marked all pieces as their own, and that they can differentiate the replicates for measurement.
Have students again measure the height and length of their pieces, with care that all their results relate to the correct pieces.
Measure: Students should record the new height and width of the pieces, and calculate individual shrinkage rates as: ( Dry Length - Bisque Length ) / Dry Length. Have students then average these rates by clay body, marking their works and results.
Constraints: Depending on kiln and class size, it may be necessary to do multiple firings to accommodate at least six cylinders per student. If this is the case, ensure all pieces from one student are fired at once, as opposed to splitting them among multiple rounds. Every round of firing is slightly variable in temperature and timing, which can alter the resultant shrinkage, confounding the final size of pieces in a non-composition dependent manner.
Step 4 - Analyze
Students should now have all their initial measurements and relative shrinkage rates. Have them collate all the data in a table and make sense of their relation to each other. Expected shrinkage rates relative to their greenware state can be found below.
| Earthenware | Stoneware | Porcelain |
Bone-Dry | 3-4% | 3-4% | 3-4% |
Bisque | 3% | 5-6% | 14% |
Key Observations: Students should see two main elements: consistency in thrown to dry shrinkage, with an increasing amount of shrinkage from earthen to porcelain. Drawing from their knowledge from the chemistry article, students will hopefully recognize the decreasing amount of fluxes in stoneware and porcelain, along with the altered bonding structure, both contributing to its increasing amounts of shrinkage.
Constraints: Depending on skill of the students and natural variability, these trends may not be accurately represented (especially in the drying step). Allow students to share their results with each other, or even calculate a class average of all data collected. The importance in this exercise is to observe relative rates-- if the trend is present but numbers are off, the lesson is still a success.
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