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What went wrong?

My gum bichromate highlights are stained or muddy

Alternative processes

What you're seeing

Whites hold a tint of the pigment colour or an orange-yellow tinge, so the print looks flat and the highlights dirty.

Check this first

Check this first

Look at the colour of the tint. Pigment-coloured means pigment stain or an unsized paper; orange-yellow means leftover dichromate.

Likely causes, most common first

  1. 01

    Staining pigment, or too much pigment

    Common
    How to tell
    The tint matches the pigment and is there in the coated but unexposed margin. Peter Blackburn: some pigments stain the paper, giving flat prints with ruined highlights.
    Test it
    Coat test strips with less pigment in the same gum and compare the margins after development.
    The fix
    Use less pigment or change pigment. Blackburn suggests beginners start with cadmiums, cobalts, ultramarines, earth pigments and lamp black.

    Sources Peter J. Blackburn / AlternativePhotography.com, An introduction to the gum bichromate process; Photographers' Formulary, Gum Bichromate Kits 07-0100 & 07-0105 instructions

  2. 02

    Paper not sized enough

    Common
    How to tell
    Pigment sits in the paper fibres and won't clear. Photographers' Formulary: without sizing the image settles into the paper and the unexposed emulsion is hard to clear.
    Test it
    Size half a sheet, coat emulsion across both halves, and expose and develop together.
    The fix
    Size the paper with hardened gelatin. Blackburn notes gelatin-sized papers sometimes need extra size hardened with weak formaldehyde or glyoxal.

    Sources Photographers' Formulary, Gum Bichromate Kits 07-0100 & 07-0105 instructions; Peter J. Blackburn / AlternativePhotography.com, An introduction to the gum bichromate process

  3. 03

    Coated paper kept too long or dried with heat

    Occasional
    How to tell
    Muddy highlights all over. Photographers' Formulary warns that emulsion left to dry too long starts to set in the paper, and hair-dryer drying can cause heat fog.
    Test it
    Coat two sheets: expose one soon after drying flat in the dark at room temperature, and the other after a long delay or hair-dryer drying.
    The fix
    Coat one sheet at a time, dry it flat in the dark without heat (the kit suggests 30 to 45 minutes), and expose promptly.

    Sources Photographers' Formulary, Gum Bichromate Kits 07-0100 & 07-0105 instructions; Peter J. Blackburn / AlternativePhotography.com, An introduction to the gum bichromate process

  4. 04

    Overexposure

    Occasional
    How to tell
    The print won't clear in the usual development time, or needs scrubbing.
    Test it
    Make a test strip with shorter exposures.
    The fix
    Blackburn: if a print won't clear in time, try a shorter exposure next time.

    Source Peter J. Blackburn / AlternativePhotography.com, An introduction to the gum bichromate process

  5. 05

    Leftover dichromate stain

    Occasional
    How to tell
    An orange-yellow tint rather than the pigment colour.
    Test it
    Soak a dry offcut briefly in a weak potassium metabisulphite solution. If the tint goes, it was dichromate.
    The fix
    Clear the dry print briefly in 5% (or weaker) potassium metabisulphite, as Blackburn describes.

    Sources Peter J. Blackburn / AlternativePhotography.com, An introduction to the gum bichromate process; Photographers' Formulary, Gum Bichromate Kits 07-0100 & 07-0105 instructions

Avoiding it next time

  • Size paper well and pre-shrink it.
  • Coat one sheet at a time and dry it without heat.
  • Wear gloves with dichromates. Potassium, sodium and ammonium dichromate are on the UK REACH Authorisation List with a sunset date of 21 September 2017, so check HSE guidance before buying or using them.

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