Washing Soda Stalactite-Cave Pillar

This crazy cave pillar forms in days, not centuries

If you've ever ventured into a naturally formed cave, you probably saw pillars stretching from the ceiling to the floor. They're called stalactites and they take hundreds and thousands of years to form as precipitation drips from down from the cave's ceiling. In the Washing Soda Stalactite experiment, you'll cut the stalactite formation process down to just a couple of days.


  • Washing soda
  • Hot water
  • Large washcloth
  • String
  • Scissors
  • Spoon
  • Small plate
  • Two beakers or glasses


  1. Start by filling two beakers with hot water. Leave about an inch at the top of your beakers to avoid spilling the water during the rest of the experiment.
  2. Add approximately 1/2 cup of washing soda to each beaker and stir both beakers until all of the washing soda is dissolved.
  3. Fold the washcloth in half, making a triangle, and roll it up tightly.
  4. Use scissors to cut three lengths of string and use the string to tie the washcloth on both ends and in the middle. Once the washcloth is tied, trim the excess string off of your knots.
  5. Put each end of the washcloth into its own beaker. Make sure that the ends of the rag touch the bottoms of the beakers.
  6. Pull the center of the rag down so that it hangs towards the table.
  7. Put the plate under the center of the rag and let it sit for three to five days.
  8. Check back during the waiting period to see the progress and watch as the stalactite forms. After the waiting period is up, you should have a pillar that stretches all the way from the washcloth to the plate!

How Does It Work?

To start the stalactite formation, the washcloth has to transfer the water and dissolved washing soda from the beakers to the plate. This principle is called capillary action. Capillary action is the same process that enables plants and trees to transfer water and nutrients from the ground up through their roots and trunks and into their leaves, branches, flowers, and fruit. The washcloth uses capillary action to soak itself in the washing soda solution until it's too saturated to hold any more liquid. Once the washcloth is over-saturated, the excess washing soda solution begins to drip from the lowest part of the washcloth, right over the plate.
When you add the washing soda to the beakers, you stir it into hot water. Hot water is more accepting of additives like washing soda, so you can mix more washing soda into hot water than you can into cold water. As the water cools and drips from the washcloth, it wants to get rid of some of the washing soda and leaves it attached to the washcloth or deposits it onto the plate below. Eventually, the excess washing soda forms a complete pillar.