Dyes vs. Pigments Re-visited

There is a common misuse of the word “dye” in the leather repair and restoration business.  While I’ve discussed this before here, it’s time to address the issue again. 

There are two methods for coloring leather.  1. Dyes.  2. Dyes and then pigment coated.  It is very rare that leather would be pigment coated without having been dyed first.

Starting with the basics – Animal skins are tanned.  This process converts the skin to leather.  The main purpose of tanning is to preserve the hide.  It stops the natural degeneration or rotting process.  At the end of the tanning procedure and before the color step, the hide is called a “crust.”

The crust is highly absorbent. Think of a chemise. The crust is infused with a dye which we all know to be a coloring element.  The actual dye molecule is very small.  It penetrates into the fiber structure of the crust (leather) and establishes the color.  Typically it penetrates completely through the leather (struck through) so looking at a cross cut, you see the same color from front to back.  The dye is not molecularly bound to fiber structure, rather it as floating within the fiber bundles.  Because it is free floating, one of its attributes is migration.  It will transfer or move. 

Water will accelerate migration.  Consider blue jeans.  They fade when washed.  The dye molecule migrates out of the medium (in this case denim) and is flushed down the drain.  I have had occasion to witness this phenomena with leather many times.  A damp white cotton cloth wiped across dyed leather will pull the color.

Dyes have a unique beauty.  Because of their small molecular construct, dyes are translucent.  You look into the leather to see its color.  It accentuates the natural beauty of the leather. Because the porosity of leather is inconsistent, some areas of the hide will accept more dye than others.  This creates the natural mottling affect you see with dyed leather.  Its beauty can’t be beat.  We classify this leather as being “unfinished.”

There is a dark side.  The dye molecule does not tolerate UV light very well.  That wave length or spectrum of light hits the dye molecule and breaks it up.  This process gradually leaches the dye from leather causing the leather to lose its color (fade).  Furthermore, dyed leather continues to have a high level of porosity.  Spill a liquid and it will soak into the leather, potentially staining the leather.  In reality the stained area has been re-colored.  So trying to clean it is like trying to clean a tattoo from your skin.

Bottom line: dyed or unfinished leather is beautiful when new, but it is aesthetically vulnerable to staining and fading.  Only about 15% of all leather furniture is unfinished.  It is typically the most expensive leather as only the finest hides (least flawed with unsightly hide characteristics) can qualify to be unfinished.

Most leather then goes through a secondary coloring process with the application of a pigmented coating.  The pigment molecule sits on the leather’s surface.  As a coloring element the pigment molecule is a big, robust molecule with excellent covering power, like snow on the ground.  The pigment molecule is carried in a binding chemistry that locks it in place.  That binder chemistry is uniquely engineered for leather.  It establishes a film on the leather surface that is opaque.  The color you see is from a topical colorant that is a pigment.  This is known as “finished” leather.  

Pigments lack translucency so the color is flatter than dyes.  But pigments are far less sensitive to UV so they don’t fade nearly as radically.  Consider and automobile leather car seat. They don’t fade despite tons of sun exposure.  They are colored with a pigment.  Additionally, the film of color on the leather will resist absorption. If something spills, you can wipe it off the leather as it won’t immediately soak in.

You can not successfully re-dye leather using dye as the coloring element for a whole bunch of technical reasons, not the least of which is that you will be wearing the color on your clothing if you sit on the furniture.  Setting the dye so that it doesn’t transfer easily can only be done at a tannery under very specific and controlled processes.   

Leather can be re-colored, but only with a pigment application.  If it was dyed (unfinished) leather in the first place, the re-coloring process is with a pigment that provides full covering power, thus converting the leather to a “finished” or pigment coated status.

Of course we do this color restoration process professionally all the time, and it is the essence of the Do-It-Yourself kits offered by Advanced Leather Solutions.  Taking it one step further, because it’s an opaque chemistry, the color coating can be changed to whatever color desired.

The next time a leather technician says he/she can re-dye your leather, be weary as that person does not understand the fundamental difference between a dye and a pigment.

Copyright  2010, Kevin Gillan