An antique armchair can add beauty and charm to any room. Unfortunately, with time the chair can become less stable. This ends up becoming less functional and more decorative. If you don't want to have to keep reminding your guests to not use it, here is how you can reinforce the chair to make it safe to use again.
Step 1 - Check the Legs
If the chair is simply wobbly, you can fix this without major issue. Measure the length of all the legs to ensure one hasn't worn down. If the chair has sliders on the legs, check to ensure the sliders are all intact. Also check the chair joints to ensure they are all intact. If not, these are all easy fixes that will only take a few minutes. To install new sliders you simply need a screwdriver and possibly a drill. Remove the old sliders, place the new ones, and drill them in place. If the old holes from the previous slider are too large, you can fill these in with wood glue and create new holes by placing the slider on a different position.
Step 2 - Check the Hardware
Since you have the chair flipped over to check the legs, go ahead and check all the hardware as well. If you see brackets or fasteners that are loose or damaged, go ahead and plan on replacing them. You should only need a screwdriver to do this on older chairs.
Step 3 - Build a Cross Brace
If the bottom of the chair is old and worn, you can easily remedy this by building a cross brace. You will need enough room under the chair to attach either 2X6 or 2X4 lumber. Measure both the width and length of the chair, and the depth you have to work with. Purchase L brackets to attach the lumber. The L-brackets will allow you to attach the cross brace and minimize any modifications you need to make to the chair. This can minimize the value of the chair, so just be aware of that. If you don't plan on selling the chair, the modification will be a welcome addition for anyone who wants to actually use the chair again.
Step 4 - Check the Webbing
If you can remove the cushion from the chair, you will be able to see the webbing that is used. The webbing is designed to create added support between the cushion and the seating surface of the chair. Antique chair webbing is going to be frail, and likely falling apart. The material used in older furniture was not meant to withstand decades of use. You can purchase new webbing that is heavier, and simply attach it to the frame with a stapler. New furniture webbing is also graded, and you can find webbing material that is guaranteed to last for 10 or more years. Be sure you pull the webbing taught as you install it for maximum security.