Lac Insect Dye in Oriental Rugs

First half 20th Century Mashhad Rug

I came across a recipe for using Lac as a dye in a book from 1907. It helps me to understand things I have observed about Lac without truly understanding. Let us look at the dye Lac as I understand it:

Lac is one of the oldest insect dyes used in Oriental rugs. Mark Whiting in Hali Magazine (Vol. 1. 1978) wrote that the insect dye lac was the principal red dye used in classical Persian carpets. Of 20^th century production it is a defining feature of Mashhad rugs.

Lac traditionally was raised in Northern India and is a  resin secreted by the lac insect (Laccifer lacca) upon the smaller branches of trees, including the soapberry, acacia and the fig tree. Lac takes it's name from the word for 100,000 - lahk which refers to the huge number of insects who are needed to make marketable quantities of Lac. Stick lac is when the lac is still on the branch and grain lac is when it has been crushed and washed. 

Use of lac yields a "cool" bluish red rather than the brilliant insect reds in the blue tones that we associate with cochineal. I have had a distinct impression that Lac dyed wool is not as long wearing as madder dyed wool and more prone to breakage. Frankly I assumed that it was akin to the fragility of shellac 88 rpm records that I remembered from my childhood. If you remember if you dropped a vinyl LP it might scratch but it was unlikely to break. Shellac records on the other hand would shatter if dropped. So when I saw the propensity to wear more quickly I assumed it was an attribute of the shellac. But then I found this recipe from 1907:

Lac dyed Mughal Carpet

Lac Red for Wool.

For 40 pounds of goods make a tolerably thick paste of Lac dye and sulphuric acid and allow it to sit for a day. Then take tarter 4 pounds, tin liquor 2 pounds 8 ounces and 3 pounds of the paste. Make a hot bath with sufficient water and enter the goods for ¾ hours; afterwards carefully rinse and dry.
From

Henleys' Twentieth Century Book of Recipes, Formulas and Processes,  edited by Gardner Dexter Hiscox

The Book is also known as:

Henley's twentieth century formulas recipes and processes containing ten thousand selected household and workshop formulas recipes processes and money-saving methods for the practical use of manufacturers mechanics housekeepers and home workers. By Gardner Dexter Hiscox, editor. 

The use of Sulfuric Acid in the dye bath suggests to me that the shorter life of Lac dyed wool may have more to do with the Sulfuric Acid them with the Lac.