How do I drill a drainage hole in a pot without cracking it?

The two things that crack a pot are heat and point loading. A dry bit spinning fast builds enough heat to fracture fired clay, and a pot resting on a hard edge concentrates the drilling force into one spot. Fix both and the job is straightforward, even on a glazed pot.

Safety

Step by step

1. Choose the bit for the material.Terracotta and glazed ceramic: a diamond-grit hole saw of 10 to 20mm, or a spear-point tile and glass bit. Stoneware and porcelain: diamond only. Plastic: an ordinary HSS twist bit. Metal: an HSS bit, ideally with a pilot hole. Concrete: a masonry bit in hammer mode is fine.
2. Bed the pot down so it cannot rock.Turn it upside down on a bed of sand, a folded towel or a stack of newspaper on a solid surface. The whole base wants even support. A pot balanced on its rim on a workbench will split.
3. Mark the hole away from the centre if the base is thick.Many pots have a raised foot ring and a thicker centre. Drill inside the foot ring but off-centre, where the clay is thinnest — you can see this by looking at the profile of the base.
4. Break the glaze at an angle, then come upright.On a glazed pot, start the bit at about 45 degrees so it bites the glaze rather than skating across it, then bring it vertical once you have a shallow crater. A ring of masking tape also helps the bit find its place.
5. Run slow and wet.Lowest drill speed, no hammer action, and a constant trickle or a puddle of water on the spot. Water carries the heat away and floats the dust out of the cut. Stop and re-wet every 20 seconds or so.
6. Use the weight of the drill and nothing more.Pressure does not speed up a diamond bit, it just heats it and cracks the pot. Let the abrasive cut. A 15mm hole in glazed ceramic can take two or three minutes and that is normal.
7. Ease off completely as the bit breaks through.Almost all breakage happens on the last half-millimetre. Reduce pressure to nothing and let the bit find its own way through the back face.
8. Drill several small holes rather than one large one.Three or four 10mm holes drain better than a single 25mm hole, are far less likely to split the pot, and are less prone to blocking.

Tips

• A plastic pot is easier if you make a small pilot hole first with a hot skewer or a bradawl — twist bits skate on curved plastic.
• If the pot is genuinely irreplaceable, do not drill it. Plant into a plastic liner pot that sits inside, and lift the liner out to water and drain.
• Cordless drills are better than corded for this because the low speed setting is easier to hold steady, and there is no cable near water.

Common mistakes

• Using hammer action on ceramic — It is the fastest way to shatter a pot. Hammer is for masonry and concrete, never for fired clay, porcelain or glass.
• Drilling dry to avoid the mess — Heat is the main cause of cracking, and dry drilling also puts respirable silica dust straight into the air. Water solves both problems.
• Drilling the very centre of the base — That is usually the thickest point and, on a footed pot, the least supported. Off-centre inside the foot ring is thinner and better backed.

Questions people ask