Molding filters
Clay pots can be made from locally procured clays or commercial products and crafted using a potter's wheel or by hand. To set the clay permanently, it must be fired. Firing requires a kiln. Since these requirements are often beyond what a local school may be able to facilitate, the student must use a model or substitute for this process which is the reason for using plaster pots in these investigations.
Making and Testing Plaster Pots
Given below is a process that can be used to study the function of the clay filter using common casting plaster or Plaster of Paris. The resultant pots can be tested for their percolation rates, degree of filtration turbidity, removal of noxious odor, and the relative number of bacteria they prevent from entering the water supply. For a discussion of Plaster of Paris and an alternative, Portland Cement, click here.How Plaster Pots Work
Plaster is normally porous when set, but not effective as a filter. In order to make the plaster porous, tiny holes need to be made in the matrix of plaster molecules. Clay pots depend on mixing the wet clay matrix with organic material such as pulverized rice hulls or wheat chaff. When the clay pot is fired, the organic material is burned leaving a tiny hole in the clay. The size of the hole is directly related to the size of the organic particles in the mix.
For plaster pots to work we must introduce tiny holes. Rather than burn out organic material, we will add holes chemically. A simple reaction works well to add microscopic holes in the plaster as it is curing. Baking soda and Cream of Tartar are added together to develop carbon dioxide bubbles in the wet plaster mix, thereby making a pot with holes. See Baking Soda for more details on this reaction.
Follow the directions below to cast plaster pots to create water filters for study:
Materials
You will need the following items/materials to conduct these tests:
• Baking Soda
• Cream of Tartar
• Plastic drinking cups
• Paper drinking cups
• Plaster of Paris
• Mixing Spoons
• Triple beam balance (there is a simple work-around if you do not have one)
• Graduated cylinder or a way to measure 60 ml of water
• Tablespoon measure
• (1/4 teaspoon measure optional if no balance available)
Step 1
Measure out Baking soda and Cream of Tartar in increments of portions of grams. The picture below shows one way to do this by using a small paper baking cup for each chemical. In the example we initially chose, we used increments of .05 grams from .50 to .75 grams each of reactant. Measure the weight of your baking paper or other paper you choose to hold your chemical sample and then set the scale to accommodate the needed weight plus the tare.
In the example at left, the baking paper used weighed 0.23 grams. Weighing these small amounts requires some skill and patience using a triple beam balance. This may be an excellent place to reinforce the skills required for this task.Note that in this example the net mass and tare mass are both indicated.If you do not have an accurate scale, you can still do the experiments. Follow the work-around below.Step 1 (alternate)If you do not have an accurate scale, you can use volume as a way to examine the variables of making plaster pots. Measure 1/4 teaspoon of each powder. 1/4 teaspoon of baking soda is about 1.5 grams. If you take the powder and carefully separate it into four equal stacks, then each stack of powder is approximately .375 grams. Separating into 8 stacks you may be able to make .19 portions of powder. There is some error here, but satisfactory for trial examinations of the process.
Step 2
Pour 60 ml of water into each of five flexible plastic cups. Room temperature water is appropriate. Add the baking soda to each cup and stir so it is thoroughly dissolved and distributed in the water. This will assure that there will not be too many pockets of bubbles created in any one place.
Label each sample with the grams of reactant. Dry Plaster of Paris and Cream of Tartar will be added to each of these cups.Step 3
Using a tablespoon, sprinkle 6 tablespoons full of powdered Plaster of Paris atop the water. Do not stir until the last plaster is allowed to soak into the water.
The way that potters often do this is to sprinkle enough plaster onto the water until there is a tiny island of plaster remaining. In most cases this will be about 1 tablespoon per 10 ml of water, but this process is more of an art than science. As soon as the plaster is in the cup and has soaked in, then stir so that there are no lumps in the mix. It should be a smooth consistency like a milkshake.As soon as the mix is well stirred, add the appropriate amount of Cream of Tartar. Mix well. The reaction of the two chemicals will produce carbon dioxide gas and bubbles in the plaster.Step 3
When thoroughly mixed, bubbles will start to form. Insert a paper cup in the middle of the plastic cup so that it is centered. It is important that the paper cup not go all the way to the bottom, but it is also important to weigh down the cup so that it does not float.
A small wooden board placed on top of the cup or a few large washers in the bottom of the cup can keep the paper cup from floating.
Step 4
In the example below, the series of different amounts of cream of tartar and baking soda were set up from smallest number of bubbles produced to largest number. Each of the plastic cups has a paper cup held down into the setting plaster with one long board. This allowed each cup to be held at an equal height.
Step 6. Allow the plaster to cure overnight. Remove the paper cup. It may have a layer of waxy surface on the inside because many paper cups are coated with wax to make them waterproof. In the example at left, we used a paper cup that had a waxy coat. You can see the pattern of this cup on the inside of the plaster. If your filter looks this way you will need to take some sand paper and remove the waxy surface from inside the plaster mold or the plaster may not be able to let any water through its pores. The plastic cup can be gently tapped face down on the tabletop to pop out the plaster. The filter is now ready for testing. See the next explorations!
