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To secure heavy items on plaster walls, use screws that are 2 inches (5.1 cm) long. If possible, get screws that have plastic anchor attachments to help hold them in place. Use a magnetic stud finder to locate the wooden studs hidden behind the plaster wall. Try to find one at the spot where you would like to hang the object. Heavy objects will be more secure if they are held in place by screws driven into the stud.  Most stud finders will light up when they find a stud. If you find a stud, mark the spot on the wall. Then use a measuring tape to make a straight line going up the wall from this spot. Along that line, mark the height where you actually want to hang the object on a piece of painter's tape. You can hang heavy items in plaster walls without driving screws into a stud. However, you should use screws with anchors to be more secure. Use the same procedure as you would for hanging light objects. However, make sure to use a drill bit that is just a bit smaller than the width of the anchor if you are using one. For instance, a screw that is 0.25 inches (0.64 cm) wide may have an anchor that is 0.3 inches (0.76 cm) wide. Use a drill bit that is no larger than 0.25 inches (0.64 cm). If possible, just use the pressure of your hand. If the anchor won't go in that way, gently tap it with a light hammer until it is flush with the wall. Be very careful, however. Plaster is brittle and can crumble if you accidentally hit it with the hammer. If you do end up with a crack or small hole in your wall, you may be able to repair it with a plaster compound repair kit. Look for one at a hardware store. Basically, you will need to carefully smear the compound into the damaged area, smooth it out, and let it set. This should be fairly easy, but you can use the drill with a screw attachment to do it quickly. Don't drive the screw all the way in. Leave a small distance between the screw head and the surface of the wall.  Attach the hanger on the back of the object to the screw on the wall. Or, if you prefer, you can wrap one end of a piece of wire tightly around the screw head, then attach the other to the object's hanger. Check the placement of your object with a level to make sure it's even.
Use heavy-duty hardware. Find a stud, if you can. Pre-drill a hole at the spot where you want to hang the object. Tap the anchor into the hole you drilled. Drive the screw into the anchor.