This process uses a backing board with a sheet of adhesive tissue paper and then the artwork is placed on the tissue paper. With this technique we use a special machine called a press and monitor 4 things: heat, time, pressure, and humidity.
The moisture in the environment when the piece is being mounted is what we are talking about. If it is more humid in the environment, then there is a chance of blisters or bubbles forming. This is caused by moisture getting trapped between the art and the backing board.
To create the pressure, we use a vacuum press. This machine pulls out the air and moisture (as much as is possible) between the artwork, tissue paper, and backing board. Many adhesive tissue papers are heat activated but never at too high a heat. Since heat is not always safe for artwork, we (and most framers) use a very low heat, or no heat activated tissue paper adhesive.
The length of time the process takes varies and the variables are many. In general, we start with the tissue paper and the recommended time for it to adhere. But most picture framers know their mounting technique cold, but this I a good reason why experience and length in the business should be a deciding factor in picking which framer to bring your artwork to.