Gold leaf techniques for applying gold leaf to leather are different from gilding on wood or other surfaces. In the main binding, the leather is vegetable tanned and quite soft to the touch. Regarding the binding, in general, the gilt decoration seen on a modern book will be gold foil or gold leaf; Genuine gold leaf is reserved for the best quality leather bindings and gold foil for cheaper work. Both gold leaf and genuine gold leaf involve the use of heat activated adhesives, gold leaf has a layer of adhesive built into the carrier film, it has the virtue of being easy to use, simply place the gold leaf and apply your heated tools. . In traditional gilding, the hot-activated adhesive takes the form of egg whites or a shellac-based adhesive.
The adhesive is first applied and allowed to dry, and then the sheet is placed over it. Heated brass tools in the form of letters or decoration are pressed onto gold foil or gold leaf. When the excess gold leaf or gold foil is removed, the letters or decoration are gilded. If there is a secret to being successful with gilding on leather, it is to understand and work with the 3 variables involved, they are; Heat, pressure and permanence.
The heat of your tools is pretty critical to success; How much pressure you apply and how long you stay in contact with the work is also important and can best be learned with good instruction and a bit of trial and error. With other leathers, like the kind of sheepskin you might find when salvaging desk tops, I suggest using gold foil for cheaper work and 23 karat gold leaf on a paper backing for better work. Some materials are called leather simply because they are made from reconstituted leather shavings, bonded together with some kind of resin; I suggest using gold foil for this type of material.