You cannot see these as they sit inside the lead came between the glass and the heart, size usually about 4mm x 1mm .
Now this is the tricky bit to put this in you need to remember two points:
1. Cut your glass 1mm smaller on each row where you are going to put a steel.
2. Never, (no matter how easy it is) put your steel in the top of the came. If you do (and you will be tempted), water will seep in, the steel will rust and expand, and push the lead and glass apart.
The picture (above) shows the correct way.
For added strength punch a hole through the border came and allow the steel to poke through so that it is just visible through the heart