There are four basic requirements needed to freeze dry books and documents:
- The object to be freeze-dried must be frozen solid; this freezing will lock the product structure firmly into position. It is important to note that the product must be frozen, or it will not freeze- dry.
- A condensing surface must be provided which is colder than the product to be dried. This surface is typically colder than -40F. This condensing surface helps attract the vapor form the product being dried.
- A vacuum pump is used to create a very low absolute pressure. A vacuum must be provided during freeze drying, This will mechanically create a vacuum/negative atmosphere. These extremely low pressures are required to complete the freeze – drying process.
- To freeze-dry successfully you need a heat source. This can be provided in many ways such as heat coils, lights, etc. By providing heat, you promote the release of bound water from the product. Bound water is the hardest to get out of the product.
What Actually Happens
The frozen documents or keepsakes are placed in the chamber. The chamber is connected to a condenser. A vacuum pump is connected in series to the chamber and the condenser after the vacuum pump is turned on. When the inside of the camber reaches the proper pressure, moisture leaves the frozen product as a vapor and collects on the condenser. This process is called sublimation. Sublimation is the process in which a solid (ice) is transferred from a solid state to a gaseous state and then recollected as a solid on the condenser without melting or returning to a liquid. Heat is also applied to the product, which will help drive off more vapors and eliminate bound water.