Defects in Vegetable Tanned Leather

Detail, Leather RV Curtains and Shade

Here are some causes of visual defects and irregularities in vegetable tanned leather seen at the tannery.

Stains on pelts

• Black Stains
  • Iron stains, chiefly caused during transferring (iron rails, drip water), appearing after sulfide liming (iron sulfide)
  • Sodium sulfide containing water
• Blue and Red Stains
  • Damages caused in curing, salt stains, germ caused by microorganisms

Stains on leather

• Brown Stains
  • Sludgy extracts
  • Flocculations in the suspenders, which deposit in the grain
  • Air stains
  • Contact Stains (Suspender liquors)
  • Tannin migrated to the surface
  • Tannins stains before coloring
  • Burnt areas, caused, e.g., by scorching
• Black Stains
  • Iron stains caused in splitting or shaving or by drip water etc.
• Greenish Stains
  • Copper stains caused by pipes or fittings
  • Sumac stains (chlorophyll)
• Grey-Brown Stains
  • Lime stains (lime blasts) caused by exposure to air, use of hard water for washing and rinsing, lime liquor prepared with fresh water
• Dark Stains, (especially in the flanks and shaved areas)
  • Scud not properly removed
• Pale Stains, round
  • Stains caused by pools of tan or bleach liquors
• Pale Stains, oblong
  • Abrasions, especially those caused by drumming
• Pale Pinholes (pits)
  • Damages caused by curing: pits caused by overbating
• Pale Spots
  • Bleaching materials not completely dissolved before addition
• Greyish Leather Shade
  • Tan liquors containing iron; concrete pits, fittings, suspension frames
  • Iron content of plant water too high, possibly caused only by rusty pipe line or condensing water containing grease
• White spue (removable with water)
  • Salt spue, (sodium sulfate, magnesium sulfate, common salt) due to sole leather seasons or to old, worked down liquors; insufficient steeping or rinsing
• White spue (not removable with water)
  • Fatty spue, usually consisting of free fatty acids, but with solvents; melts in the heat; and sometimes of higher fatty alcohols
• Leathers match test failures
  • Fat liquored too acid or unsuitable fat liquoring agents were used. May also be caused by fat splitting microorganisms
• Uneven penetration, (untanned middle)
  • Too intensive coloring (case-hardening)
  • Sludge layer in the pores
  • Excessively exhausted liquors
• Poor and loose flanks
  • Poor raw material
  • Lime too old
  • Bating bath too strong or too warm
  • Excessive straining

Grain faults

• Loose Grain
  • Poorly cured raw material
  • Excessively stressed by prolonged soaking, liming or bating
  • rummed too long in short liquors
  • Overstrained by swelling
• High Grain
  • Excessive initial swelling in the lime
  • Coloring liquor too acid
• Cracked Grain
  • Careless handling of the raw hides
  • Careless handling of excessively swollen pelts
  • Excessive pressure of sammying and setting out machines etc.
• Pebbled Grain
  • Prolonged drumming
  • Liquor too short
  • Rate of revolution too high
• Drawn Grain
  • Coloring in liquors that are too fresh, too acid, too strong or too warm.
  • Liquor too short
  • Rate of revolution too high.
  • With combination- tanned leathers, tanning liquors too strongly absorbed

Tags: Leatherworking