Soft or not setting
- Not fully dissolved. Agar must be heated to a full boil and held for one minute until the liquid is clear. Warm water with stirring looks dissolved but leaves particles behind.
- Acidic formulations. Below about pH 5, agar hydrolyses during autoclaving and the gel weakens. Such media are usually acidified after sterilisation or formulated with more agar.
- Remelting. A set medium can be remelted once; from the second time on, gel strength drops noticeably.
- Weighing or volume errors. The standard concentration is about 12 to 15 g per litre.
- Over-sterilisation. Time or temperature beyond specification also weakens the gel.
Too hard or brittle
Too much agar, or too much water lost to evaporation during prolonged open boiling. Make up the lost volume before pouring.
Surface moisture and condensation
Pouring too hot, or closing plates before they have fully set. Cool the medium to 45 to 50 °C before pouring and leave lids ajar briefly so the surface dries.
About the agar itself
Gel strength is specified on the agar's data sheet (for example ≥ 800 g/cm²). Different grades give different firmness at the same concentration, so run a small trial when changing agar source.