What can you do with old tapes which have been affected by hydrolysis? Here's a procedure for "cooking" the tapes to enable the material on them to be copied on to other media.
What You Need
The oven we use is the oven in an old domestic stove. The element has been removed and replaced with a heater. The heater will be described later.
A hole approximately 75mm has been made in the back wall of the oven at the bottom. This is to allow air to escape from the oven. Also this hole can be used to bring the thermometer probe and the thermostat probe into the oven and take the power cord from the heater and fan out of the oven.
The choice of a heater is very important. The original heater element in the stove is unsuitable for this application, as the thermal residue in the element after it is switched off is too great. The element that is used needs to cool very quickly so as not to raise the temperature above the maximum tolerance.
The heater should be a single bar heater about 500 watts with a glass type element. The element is constructed of a glass tube with a nickel chromium wire element inside it. This is the type of elements used in most domestic heaters these days.
The fan is a 120mm Rotary Fan. These fans are used in electronic equipment etc.
We use an electronic thermometer with a long remote sensing probe. This allows the probe to be placed in the oven and the meter to be outside for easier viewing. It will measure -10 to +110deg Celsius. It is a MICRONTA DIGITAL THERMOMETER 63-9361.
The Thermostat required needs to be able to switch the AC Voltage used for the HEATING ELEMENT, e.g. 250V AC or 110V AC. The specifications on the unit we use is 5amp 250V AC with a temperature range of 30deg C - 110deg C. Two units that suite these specifications are:
- CAEM Mod TU 16 (5)A 250V AC 30deg - 110deg C.
- PRODIGY TR-86 30deg - 120deg C.
The Differential temperature (the temperature range required to switch the thermostat off and on) needs to be very small. A thermostat with a Differential temperature of a couple of degrees Celsius should be chosen. LRI Australia would be able obtain the thermostat if required.
These rings are used for separating the reels of tape during the drying stage. They are about 45mm dia X 15mm high.
The purpose of these cans is to cool the tape in an atmosphere that is free of moisture.
Note: If you have a room set up for archiving that is temperature and humidity controlled the tapes can be placed in the room to cool.
The cans we have used are round biscuit cans that are a little larger in diameter than 7" dia spools and they can hold about four tapes. They have a tight fitting lid.
These cans need to have a layer of SILICA GELL. The LAYER needs to be at least 6.5mm thick. An INDICATOR SILICA GELL is best because this allows you to monitor how much moisture is in the GELL. When the GELL becomes too saturated it can be dried out in an oven at about 110deg C. When the GELL returns back to its original colour it is ready to be used again.
Note: It must be put in a sealed can and allowed to cool. If it is allowed to cool in open atmosphere it will absorb moisture very quickly. A layer of cloth preferably cotton cloth is laid on top of the SILICA GELL. This is to prevent the GELL from coming into contact with the TAPES. Mark the cans to indicate that they are to be kept flat.
Setting up the System
The HEATER and FAN are placed in the bottom of the oven. The electric cords for both of these can be taken out through the hole in the back of the oven.
The THERMOSTAT and THERMOMETER probes are placed on the inside side wall of the oven about half way up the wall. We stick the probes to the wall using some "Blue Tak". The main bodies of these two units are located outside the oven.
The FAN is to run at all times during the drying time.
The HEATER will be switched on and off by the THERMOSTAT.
- The OVEN is preheated until the temperature stabilizes.
- Adjust the THERMOSTAT until a temperature range of 51deg C to 57deg C is achieved. In practice this temperature range would be translated as follows.
When the THERMOSTAT switches ON the OVEN temperature would be about 51deg C. After the THERMOSTAT switches OFF the temperature will continue to rise for a short time. The residue heat left in the element causes this. The 57deg C temperature is the temperature after the THERMOSTAT has switched OFF and the temperature in the oven has stabilized.
Note: It is most important not to allow the temperature to rise above the 57deg C limit. If necessary it would be better to adjust the THERMOSTAT to allow the lower temperature to be lower then 51deg C to make sure the higher temperature does not go above 57deg C.
- The end of the tape must be taped down very securely. The fan causes a lot of air to circulate in the oven. If the end of the tape comes adrift it will be sucked into the fan and damaged. The choice of adhesive tape is very important. The temperature in the oven will be above 50deg C. Because of this it is necessary to use an adhesive tape that will not become unstuck at this temperature. We use a type of masking tape that is used for high temperature applications - the particular brand is TESA 4330.
- The TAPES are placed in the oven separated by the METAL RINGS.
- The TAPES are cooked for eight (8) to twelve (12) hours.
- When the tapes are removed from the oven they must be immediately placed in cooling cans. As the tapes are cooling they will absorb moisture very quickly if they are allowed to cool in open atmosphere.
- Because this treatment is A TEMPORARY FIX AND NOT PERMANENT the tapes need to be copied immediately after treatment. DO NOT treat more tapes then can be copied within a few days. Leave the tapes in the COOLING CANS until they are ready to be copied.
- The recommended tape for copying the treated tapes onto is BASF 911.