If you want to preserve hydrangeas' large blooms for an indoor bouquet that will last a long time, there are a couple of ways to do so. First, you can just dry the flower. Hydrangeas are flower that lend themselves well to drying. If you want a softer, suppler bloom, you can also dry hydrangeas with water and glycerin.
The simplest way to dry a hydrangea bloom is to remove the leaves along the stem, group five or six flowers together with a loose rubber band or some string, and hang them upside down to dry in a cool, dry place.
Another way to dry hydrangeas, although it seems counterintuitive, is to put them in a vase with a couple of inches of water. Place the vase out of direct sunlight, and when the water evaporates, add more. Continue doing this until the blossoms are suitably dry.
To dry them with glycerin, you will need glycerin—available at drugstores—and some water. You will also need food coloring as this process turns the blooms a golden brown. Cut the stems after the flower has passed its peak. Fresh blooms will wilt and turn brown. Cut stems no longer than 18 inches, and make the cut on an angle. Crush the stems a bit with a hammer to make it easier for them to draw up the solution.
In a vase, mix two parts water to one part glycerin. Add a drop or two of food coloring if desired. Put the stems in the solution. The mixture will be drawn up into the flowers where the water will evaporate and the glycerin will remain in the petals. It will take two to three weeks for the flowers to dry completely using this method.