My preferred anchors in drywall for heavier loads are the newer toggle strap types.

These have a steel brace that rests inside the wall, spreading te load over about 3 inches. They also have the benefit of allowing you to remove and reinstall the bolt, which older toggles didn't allow.

The toggle is installed through a hole, usually 3/8 or 1/2 inch. The straps are slid to rotate the toggle bar until it is perpendicular to the wall. the Toggle is insterted and the straps are then slid back turning the bar parallel to the back of the drywall. The straps are pulled forward and the collar is pushed in to the face of the drywall, locking hte toggle in place. The excess straps are then broken off, and the toggle is ready for a bolt. (It reads loger than it takes.)

Rigid Attachment

You can drill holes in the mirror frame and into the wall to mark where the toggle should go. Once the toggle is installed, you can bolt throught those holes to lock the mirror to the wall. If the mirror is resting on the floor, only two toggles should be needed near the top.

Loose Attachment

You also could attach the mirror to the toggles using heavy duty braided picture wire. Insert two heavy hookeyes into the back of the frame. You can make an attachment point for the wire at the toggle bolt using a piece of perforated metal strapping, or an S hook bent to close the loops. Put the bolt through this attachment point and into the toggle. Tighten. Then loop the wire through the wall attachment point and the hookeys and close the loop with several twists of wire.

This second method is not meant to hold the mirror tight to the wall, just to provide a leash if it were to tip forward. Again, two toggles should be sufficient.

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