Wax discs were traditionally used in home canning to create a seal on the surface of jam or preserves. However, they haven't been commonly used for many years. There are several reasons why wax discs are no longer recommended or commonly used for jam making:

1. Ineffectiveness: Wax discs are not as effective at creating a proper seal as modern canning methods. They can allow air and contaminants to enter the jar, leading to spoilage and potential health risks.

2. Mould Growth: Since wax discs do not provide an airtight seal, they can allow mould to grow on the surface of the jam. This can result in spoilage and render the jam unsafe to eat.

3. Contaminant Risk: Wax discs can introduce unwanted contaminants into the preserved food. The wax itself might not be food-grade or could contain impurities that could leach into the jam.

4. Limited Shelf Life: Jams preserved with wax discs have a shorter shelf life compared to those processed using modern canning methods. The lack of a proper seal can lead to the deterioration of flavour, texture, and colour over time.

5. Modern tinplate lids are manufactured with a PVC coating and seal which protects the metal from any acidic effects and creates an air tight seal.

6. Health and Safety Concerns: Using wax discs might not meet current food safety standards. There is a risk of bacterial growth and other foodborne illnesses when using inadequate sealing methods.

7. Appearance: Wax discs can affect the appearance of the finished product. Wax might not adhere evenly to the jar's surface, leading to an unattractive and uneven appearance.

Given these concerns, it is recommended to use modern techniques for preserving jams and other foods. These techniques typically involve using proper canning jars, lids, or a heat processing method (such as water bath canning or pressure canning) to create a reliable and safe seal. If you're interested in preserving jams or other foods, consider following guidelines from reputable sources such as the local food safety organizations to ensure the safety and quality of your preserved products.