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Chrome Alum Gelatin

Chrome Alum Gelatin

Subbed Slides

The term subbed slides is often used to refer to slides coated with a mixture of chrome alum and gelatin. At other times it is used to refer to slides coated with any adhesive material that are prepared ahead of time. Here, the term will be used to refer to slides coated with chrome alum gelatin ahead of time.

Gelatin forms a gel with chromium, but it retains its stickiness while adhering well to the glass of the slides. Kiernan explains the chemistry behind this as the chromium covalently bonding to the surface of the glass and to some proteins in the section. This strongly attaches the tissue to the slide. Dried slides can be stored for a few years without deterioration.

Subbing may be done in batches or on individual slides. In either case the chrome gelatin mixture must be reasonably fresh, and preferably made just before use. This ensures that bacterial contamination is kept to a minimum. Care must be taken to ensure that a thin, even coating is applied. To this end, slides prepared in batches should be drained with the holder placed at an angle and collected solution absorbed from the corners with a suitably absorbent material.

Individually subbed slides are easy to prepare. A small quantity of the chrome gelatin is dabbed onto the slide and spread evenly over it. They are then allowed to dry. The requirement that subbed slides must be dried means they have to be prepared beforehand, not during sectioning.

Fol’s chrome gelatin was recommended for free floating and celloidin sections in water, although it likely works equally well for the usual subbing of slides. Again, individual slides or batches of slides in a rack may be subbed.

Cleaning Slides For Coating

Slides must be clean and the following is recommended before coating:

  1. Clean slides thoroughly with a detergent and water.
  2. Wash slides thoroughly with water to remove all traces of cleaning solution.
  3. Rinse well with distilled water.
  4. Dry the slides.
  5. Continue with the procedure for coating.

Subbing Slides in Batches


1% gelatin1volume
0.1% chrome potassium sulphate1volume


  1. Immerse a rack of slides in the chrome gelatin solution and agitate up and down a few times.
  2. Remove from chrome gelatin and drain, ensuring slides are evenly coated.
  3. Dry, warming if necessary.
  4. Store in a refrigerator.

Fol’s Chrome Gelatin


Glacial acetic acid20mL
Ethanol, 70%280mL
Chrome potassium sulphate, 5% aqu.4-8mL


  1. Dissolve the gelatin in the acetic acid.
  2. Dilute with the ethanol.
  3. Add the chrome alum and mix well.
  4. Coat the slides according to the subbing instructions above.

Safety Note

Prior to handling any chemical, consult the Safety Data Sheet (SDS) for proper handling and safety precautions.


  1. Kiernan, J A., (1999).
    Strategies for preventing detachment of sections from glass slides.
    Microscopy Today v. 99-6, p. 22-24.
  2. Gatenby, J. B. and Beams, H. W., (1950)
    The Microtomist’s Vade-Mecum Ed. 11. p. 117.
    J. & A Churchill Ltd., London, UK.