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This looks like a perfect project for slip casting (a one-piece mold would be a snap with our 3D printing techniques). You can control the water permeability by both temperature and wall thickness. Our RedArt terracotta recipe L4170F would be a good starting point. It is 90:10 Redart and OptiKast kaolin. These are sold of Amazon as "Terra Cotta Water Spikes", getting a few of those for testing might be a good way to start making your own.
Richard notes that the thickness of the wall doesn’t seem to affect the migration nearly as much as the firing temperature. He recommends starting with cone 06. To achieve more permeability, fire lower (this will be at the expense of strength and durability). For less permeability, fire higher or add a little frit to the recipe (e.g. 3% frit 3110). Richards notes there are other variables too: Humidity, type of soil and how established the plant is. He says the roots will grow right onto the spike once watering is established.
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