Experiment 1


Clay Labs


To compare mechanical and thermal properties of fired and unfired beams made from art clay and clay suspensions (slip).

Review of Scientific Principles:

Clay was the first ceramic material used by humans, and it continues to be useful in modern times. Clays used for pottery are composed mainly of hydrated silica (SiO2) and alumina (Al2O3). Small amounts of other minerals (Fe2O3, MgO, etc.) are typically present.

Clay is somewhat unique in its ability to be plastically formed (shaped) when wet. This plasticity depends on the amount of water, the size and shape of the particles, ionic content, and temperature.

Clay slip is made by mixing clay with water to make a mixture that can be poured into a mold. This method, called slip casting, is used to make thin, detailed products. Plaster of Paris is commonly used to make the molds because it is inexpensive, easy to work with, and highly porous (easily absorbing water from the cast slip).

Clay objects must be allowed to dry before firing to eliminate most of the pore water. The remaining pore water is eliminated during the initial stage of firing at around 100 ° C.

Firing and sintering change the properties of the object significantly. At about 350 ° C the water of hydration is driven off. As the temperature increases into the sintering stage, the porosity changes from an open to closed network; and the object shrinks as porosity is eliminated. This leads to increased density and improved mechanical strength. The fired product is hard, dense, more durable, impermeable to liquids, and brittle. This activity investigates the relationships between mechanical strength, density (porosity), and thermal conductivity of unfired and fired clay objects.


The slip casting method is used to make a variety of ceramic objects (e.g., clay-based dishes, kitchen and bath fixtures, as well as silicon nitride (Si3N4) turbocharger rotor blades). It is useful for three-dimensional complex objects with uniform wall thicknesses less than two centimeters.


This lab takes several days (6) to go from preparation to testing.

Part A (make molds):1/2 hour first day
Part B (make beams and fire):1/2 hour second day to make a set of beams of each type, 3 days for clay slip to air dry before firing. Firing time varies
Part C (Test beams):Testing four beams requires about 20 minutes for each test

Materials and Supplies:

Part APart BPart C
Plaster of Parissoft clay 3-point test apparatus
form (container) for plasterclay slip2 beams (green and fired)
block form for beamwaste bucketcandle
spoonruleriron ring
waste bucketbalancenails (4)
spoonkilnring stand
knifegas burner
All parts: safety glasses and apron

General Safety Guidelines:

*Plaster of Paris, clay, and clay slip are very safe to use.
*Do not wash any of these down the sink! They can solidify and clog the sink. Use a wash/waste bucket that can be dumped outside.
*Be aware of the high temperature of the kiln and the possibility of burns from that source.


Part A: Preparation

1. Mix Plaster of Paris and water in the specified container, making enough to partially fill the form to a depth of 3-5 cm with a mixture that has the consistency of pudding or yogurt. Smooth out the top surface of the Plaster of Paris mixture by tapping the mixture on the table. Work quickly. You have less than five minutes after adding the water before solidification sets in.

2. Push the block form down into the plaster to a depth of about 1 cm. Hold the block in place for a few minutes until the plaster begins to harden. (Note the change in temperature as the plaster hardens.)

3. After the plaster has set, remove the block form from the mold and let dry overnight.

Mold Diagram

Video Clip

Part B: Making Beams

4. From the art clay, make a beam identical in size to the one that will be made in the plaster of paris mold.

5. Record the beam's mass, length, width, and thickness.

6. Pour clay slip into the mold to a depth of 1 cm. Let stand until it is hard enough to remove from the mold.

7. Remove the beam from the mold. Measure its mass, length, width, and thickness. Record. Calculate the beamÕs density.

8. Allow the beams to dry for at least three days before firing. Before the firing is done, measure the mass and dimensions of the beam. Calculate the density of the beam.

Video Clip

Part C. Testing Of Beams

10. Before testing, measure the mass and dimensions of each beam. Calculate the density of the beam.

11. To test for the thermal properties of your beam, use a lit candle to drip wax on the beam. Attach four nails equally distributed along the side of the beam. Set the beam on a ring stand. Heat one end of a beam with a gas burner. Time how fast the heat travels down the beam by watching the objects fall when the heat reaches them. Record the times.

Video Clip

12. To test mechanical properties, place the beam to be tested across the supports. Attach any equipment needed for the testing apparatus.

13. Attach a container and add mass until the beam gives way.

14. Measure the added weight. Calculate the force that broke the beam.

15. Clean up as directed by instructor.

Video Clip

Data and Analysis:
MeasurementGreen clay beamFired clay beamGreen slip beamFired slip beam
date formed
date fired
mass 1
mass 2
mass 3
dimension 1
dimension 2
dimension 3
density 1
density 2
density 3
nail 1
nail 2
nail 3
nail 4
added mass
force applied


1. What is accomplished by firing that is not accomplished by simple drying?

2. What might happen if the beam were fired before it dried?

3. How is strength different from hardness?

4. Summarize the differences in density, mechanical and thermal properties between the fired and unfired beams as observed in this lab.

5. Palette, the art teacher, fires an assortment of ceramics. What might happen if the firing temperature was too low? What if it was too high ?

Teacher's Guide to Experiment #1 Clay Labs

Consult the art teacher or ceramics craft shop for firing times.

Materials and Supplies:

*Plaster of Paris may be purchased at some hardware stores. Soft clay (such as is used on a potter's wheel) is available in art supply stores. Do not use the plastic, nonbaking type of clay. Clay slip is available from ceramic craft shops. Both clay and clay slip can be purchased from American Art Clay Co., 4717 W. Sixteenth St., Indianapolis, IN 46209-2292.

*It is recommended that lab tables be covered with plastic or newspaper to simplify clean up.

*A three-point apparatus is designed to support the beams at the ends while applying a force in the middle. This apparatus can be a simple as 2 desks to support the beams and a rope loop in the middle on which to hang weights or a bucket to hold weight.

*A second class lever system could also be set up.


*This procedure is written for each student group to produce one Plaster of Paris mold, one formed clay beam, and one poured clay slip beam. If done this way, two groups will have to work together in Part C with one set of beams having been fired and the other set left green. It is important that all beams be nearly the same size. If variations are desired, adjust time and quantities of materials used.

A-1 *Plaster of Paris may be mixed in mold form or in a separate container and then poured into the mold.

A-2 *The block could be removed shortly after the Plaster of Paris begins to set, or it could be left in the form until the next day.

*A piece of 2" x 2" or 2" x 4" lumber or 1" x 2" firing strips or a plastic form can be used as a block form. These could be marked with a line at 1 cm to get consistency in depth among the student groups. If other objects are used to press into the plaster, they may need a light coating of oil to prevent them from sticking to the plaster.

B-4: *There are a couple of ways that this can be done. One is to press clay into the mold, smooth, and pull out a beam. You may have to run a knife around the edge to help remove the beam. Or the block form could be used as a template to cut a beam from a slab cut off the stock clay (try to cut off slabs to just the right thickness) or rolled to the right thickness. Test tubes or graduated cylinders make nice rolling pins.

B-6: *This might be about 30 minutes or more. The poured beam could also be left in the mold until the next day .

B-7: *Students are instructed to find the density through mass and linear measurements of the beams when they are first made, after air drying, and after firing. Additional measurements could be added to get a more detailed picture of the changes that occur during these processes.

B-9: *If a large ceramic kiln is used, it may take 20 minutes to load the class items into the kiln and several hours to bring the kiln up to temperature and then overnight for it to cool back down. If a small enameling kiln is used, the process may take only an hour or so.

*It helps if the beams are prewarmed and dried at about 200 ° F in a regular oven for 2 to 4 hours before firing.

*The beams to be fired are to be dried, either for several days in a warm dry place or several hours in a drying oven at 100 ° C.

*Put the object in a kiln and the raise the temperature slowly until the maximum temperature called for is reached and held for several hours.

*Turn the kiln off and allow it to cool before opening. If a ceramic kiln is not available, it is possible to use an electric hot plate with an 8 inch clay flowerpot lined with aluminum foil and inverted on the plate. This small kiln will give a temperature around 1300 ° F. This really works!

C-12: *If the 2nd class lever system is being used, then measure the basic down force by hooking a spring balance to the end of the lever arm. If a bucket on a rope is being used, make sure they are the same for each test group or record the different masses.

C-14: * If a second class lever was used, add the basic force of the lever arm and this is your effort force. Multiple this force by the IMA (Ideal Mechanical Advantage) of the machine. You will have the force that was needed to break the beam.

*You may want to do a 3-point test or thermal lab on other materials like glass (Pyrex), plastic, and metals for a materials comparison.

Answers to Questions:

1. Firing fuses the particles together.

2. The water that was still in the pores might cause the beam to break.

3. Strength is the ability to resist deformation, hardness is the ability to resist abrasion.

4. Answers will vary. Basically unfired beams are more dense, less thermally conductive, and weaker than fired beams.

5. Too Low - objects would crumble. Too high - objects would be melted down like glass.

Sample Data and Analysis:

MeasurementGreen clay beamFired clay beamGreen slip beamFired slip beam
date formed
date fired
mass 1 85.7 g
78.7 g
mass 257.2 g
63.4 g
mass 3 51.1 g 42.8 g 51.3 g 44.7 g
dimension 1 9.5 cm 9.5 cm 9.5 cm 9.5 cm
dimension 2 3.5 cm 3.5 cm 3.5 cm 3.5 cm
dimension 3 1 cm 1 cm 1 cm 1 cm
volume 33.25 cm3 33.25 cm3 33.25 cm3 33.25 cm3
density 1 2.58 g/cm3
2.37 g/cm3
density 2 1.72 g/cm3
1.91 g/cm3
density 3 1.54 g/cm3 1.29 g/cm3 1.54 g/cm3 1.34 g/cm3
nail 1
2 min.
1 min.
nail 2
3.5 min.
2 min.
nail 3
5 min.
3 min.
nail 4
9 min.
5.5 min.
added mass3.8 kg5.5 kg3.2 kg5.7 kg
force applied37.24 N 53.9 N 31.6 N 55.86 N

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