Published on 10/13/2026 · by Leandro Caldas, glazier, founder of Routeck and creator of the Arena
How to break out a mirror without chipping: the tap from below
The first Arena do Vidro challenge started with a Hello Kitty mirror on the bench and one rule: make an exact copy in 1 hour and 30 minutes. The mirror came already scored. Isaac cut it on the Router Glass, which cuts mirror and also laser-cuts vinyl, but the contestants had to cut the vinyl by hand. Step one, before any finishing, was breaking the piece out of the sheet.
And break-out, something every glazier does dozens of times a day, is where the episode taught its first lesson. In this post I explain how to break out a mirror without chipping it, based on what Vinícius, a judge with 30 years in the glass trade, showed at the bench.
Nerves and the forgotten step
One contestant said later that nerves got to him. Under pressure, he forgot a step he always uses: knocking down the edges, the waste glass around the drawing, to make the break-out easier. He only remembered afterward. The other said that when he applied pressure he saw the score running well, and break-out was easy for him.
Knocking down the edges means removing the extra glass around the piece before trying to free the shape. The less waste glass left locked into the curve, the less force you need, and the lower the risk of the break running into the piece.
The "oyster": tapping with the cutter's ball
Vinícius explained it with a cutter in hand. It has the diamond head, which does the cutting, and the part that holds the oil. Lots of people flip the cutter over and tap the glass with the ball at the end to open the score. On a mirror, depending on how you tap, that ball catches the silvering and makes a mini oyster, a "butterfly": that little shell-shaped chip.
The problem doesn't show right away. It shows later, when you seam or edge the piece: the chip forces you to keep sanding that spot to get rid of it. That's exactly what happened later in the same episode. One contestant got stuck while edging, and Vinícius named the cause: the chip came from the tapping.
The tap from below
The technique Vinícius used is what he calls the "cascudinho", a light knuckle tap. Instead of tapping with the cutter, you use your hand, under the glass, giving a light upward tap along the score. And you keep turning the piece as you go.
- Score the whole shape before you start opening it.
- Knock down the edges: remove the extra glass around it so less waste is locked into the shape.
- Tap lightly from below with your hand, following the score and turning the piece, until the whole cut opens.
- Only once the score has run all the way around do you break the piece out.
- On a closed shape, add two relief cuts, one at each end: according to Vinícius, the glass then opens straight away and breaks out on both sides.
Done this way, depending on the template, the piece comes out whole in one go, with no chip in the silvering and no extra edging work. Later in the same episode the panel summed it up: by hand, from below, nothing would have happened.
Machine and hand: where each one helps
In the Arena, the Router Glass did the mirror cutting, and the contestants broke it out by hand. That's daily life with a CNC: the machine scores the shape precisely, but break-out is still the glazier's job. One judge noted that one contestant, not used to CNC and with a more manual process, was slower at break-out but steady. The other, who broke out first, clearly had the technique.
| How you open the score | Risk to the mirror | Effect on edging |
|---|---|---|
| Tapping with the cutter's ball | Mini chip in the silvering | More sanding to remove the chip |
| Light tap from below, by hand | Score runs clean along the line | Normal finishing |
| Forcing it without knocking down the edges | Break can run into the piece | Risk of losing the piece |
- What is the "oyster" on a mirror?
- It's the small shell-shaped chip that appears in the silvering when you tap the mirror with the cutter's ball to open the score. It means extra sanding when you seam or edge the piece.
- How do you open a mirror score without chipping?
- With a light upward tap by hand, following the score and turning the piece until it fully opens. Only then break it out.
- Do curved shapes need relief cuts?
- In the episode, Vinícius explained that with two cuts, one at each end, the glass opens straight away and breaks out on both sides. Knocking down the edges first also helps.
The clip with this part of the episode is at the top of the post, and the full EP1 is on the episode page. If you break out mirrors better than these two, applications for the next Arena season are open here on the site.
Full episode 1 · Arena do Vidro episode 1: the Hello Kitty mirror · How to cut a mirror into shapes