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

Leather Colors and Attributes

The phone rings and answered: “Advanced Leather Solutions, how can I help you?”

A common opening response is: “I have a problem. It seems the color of my leather sofa is disappearing. What’s going on and can it be corrected?”

After a few qualifying questions, a conclusion is reached. The leather is suffering from print coat failure.

To understand what that means requires some background knowledge. Most upholstery leather is vat dyed (aniline) and then a pigmented coating is applied to the surface (more on the difference between dyes and pigments in another post). The topical color presented comes from a cocktail of pigment molecules mixed to create a specific rendering (i.e. brown). The pigments are blended in a resin-based chemistry called a “binder.” The result is referred to as the colored “finish.” All of this is then sealed with a clear coat which is the primary protection.

Now, here’s the meat of the matter. The binder and clear coat are chemically engineered with several important attributes:
1. Elasticity -- they have to flex and move as the leather flexes and stretches when sat upon else they will crack.
2. Cohesion – they have to establish a durable, wear-resistant film where each molecule links or “sticks” to its adjacent molecule else the will wear quickly away.
3. Adhesion – they have to adhere permanently to the surface else they will peel up.
4. Chemical Resistance – they should withstand the rigors of an active household else will wash away, sometimes simply with water.
5. Low Profile – they should follow the topographic contour of the leather flowing down the side, across the valley floor and up the other side of the grain pattern on the hide else they will bridge over the grain pattern, obliterating it.

Without these attributes the finish will come off, peel up, crack, appear like plastic (vinyl), etc.

Base and Print

To address the issue above, there is one other variable to consider. To present a color that looks natural a layered coloring technique is used. This is called a “base and print.” The base coat (lighter color) uniformly covers all leather surfaces. The print coat (darker color) is then erratically applied without complete coverage creating a mottled coloring affect. The technique gives the illusion of color depth, or a more natural look, resembling how a dye would render in leather.

It the case of the common question mentioned above, the tannery failed with at least two attributes: 1. Adhesion, 2. Chemical Resistance. The clear coat has eroded away and the print coat has been either worn or chemically removed (sometimes with simply water), exposing the base coat. Thus print coat failure ensues exhibiting as lighter color (the base coat) in the problem area. Often the problem is described as fading.

This is a correctable condition. It requires application of the missing print coat and then top-coating with a more durable chemistry so as to avoid the same problem from reoccurring.

As professional leather restoration and repair technicians, we see common threads of weakness in leather furniture. Typically there is a failure at some level in the basic chemical composition of the finish applied at the tannery. With an understanding of the fundamentals, we can generally develop a fix that solves the problem. However, it’s important that the “fix” itself not be problematic.

Here’s the rub. There are people representing themselves as leather repair technicians who have no clue about the fundamentals and cannot distinguish the difference between quality finishes and terrible finishes.

For example, many technicians will use colored finishes that have been engineered for vinyl or plastic. Leather is a very different material as it’s organic, not synthetic. This differential is important. Vinyl colors when applied to leather often crack and peel over time. So, it’s vital to determine if the color chemistry the technician is using is specific to leather. If the technician represents that he/she uses the same color chemistry for both vinyl and leather, then that is a prescription for failure.

To learn more about leather finishes, go to http://www.advleather.com/. Click on the leather care button. There you’ll find a description of the various types of leather and their finishes. You can also call our technical staff at 510-786-6059.

Copyright 2009 Kevin Gillan