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Thanks in advance for advice.

I have a piece of deep royal purple coated with rainbow irid, fused onto clear, about 10.5" x 5". It's been slumped into a tray mold.

When I tried dremelling a 2 mm hole, at an angle, through the clear and out the irid face, the irid chipped a bit.

Does the coating just tend to chip, or is there a way to make this work well?

The irid will chip some, but it will chip even more if you drill from the back. Any time you drill anything, the smaller hole will be in the front and the back will kind of chip out.

Dana W.

Dana's correct, the chipping occurs when the bit emerges from the underside of the glass.

To minimize chipping, drill slower. Especially with a Dremel, which at top speed is way too fast for most small bits. As you approach the bottom of the hole, slow down even more. This is good advice regardless of the tool being used.

Also, you can sometimes help by putting a second, sacrificial piece of glass underneath the first one. Or by covering the bottom of the glass with a piece of duct tape.

With clear glass you can drill part of the way from one side and then flip the piece and drill the rest of the way from the other side, but it's really hard to get the holes to line up properly, and nearly impossible with opaque glass.

I have used a spade shaped bit and drill until it just barely comes thru the surface of the opposite side, then turn the piece over and drill from the other side. I've drilled 1000's of holes this way and have only had a few that cracked because I was not careful to support the glass evenly. I almost always use a drill press with a water pump to spray on the glass as I'm drilling.
One the piece that split out , you might try a spade bit and just try to round up the edges. go slow!

It's difficult to always anticipate how a piece will be finished but I try to do as much cold work as possible before and during fusing i.e. finishing glass edges after cutting and before the initial firing, shaping/finishing after initial firing and before fire polish/slumping, etc. This includes drilling holes so that a chip or break occurs before I've invested time and effort into the piece. I mark the glass so that holes can be drilled in what will be multiple, stacked layers and then drill holes partially from back and fully from front to avoid chips; stack the pieces and pack holes with rolled refractory material and fire. The holes usually need to be 'trued' by grinding once the piece is fired but damage from drilling is mostly avoided or minimized. Slower firing schedules may need to be used as the holes and refractory material may introduce firing risk (temperture differentials) but the time and potential expense saved is worth the extra firing time (at least to me).