I have a confession: researching this topic revealed how much I didn’t know. My family has made soap for five generations and primarily Castile soap for three, so I expected to be confirming familiar lore. While the chemistry of soap was familiar, the historical details of Castile soap surprised me.
This is a dive into soap history and craft—soap nerdery, if you will. I spent work hours reading fascinating historical accounts and discovered unexpected intersections between soap and larger forces: global trade and empires, class and religion, scientific discovery, and public health. It turns out soap connects to many threads of human history.
Soap is one of the oldest intentional chemical reactions still used today—second only to fermented products like beer—and remains a vital defense against illness.
Table of contents:
- Origin of Soap
- Soap for personal hygiene
- Materials used to make early soaps
- History of vegetable oil-based soap
- The advent of Castile soap
- The decline of Castile soap
- My family’s history making Castile soap
- Castile soap today
Origin of soap
The first soapmakers likely never knew they had made something lasting. Pinpointing a single moment of discovery 5,500 years ago is impossible, but we do have early written evidence. A Mesopotamian clay tablet from the 3rd century BC records a basic soap recipe, showing that ancient people understood and intentionally performed the reaction we now call saponification.
Soap forms when fats or oils react with a strong alkali. It’s easy to imagine fats from cooking or sacrifices mixing with alkaline ash from fires to produce soap. Another plausible origin is textile production: ancient weavers discovered that treating wool with ash improved dye uptake, possibly because the ash reacted with lanolin to create a rudimentary soap-like substance. Both scenarios are sensible, but we may never know which came first.
By 2500 BC there are references to soap being used in southern Sumer for fabric production, medicinal preparations, and ceremonial washing. Regular personal hygiene as we understand it today was a later development.
Soap for personal hygiene
Soap’s primary uses in antiquity were industrial, medicinal, or ritual rather than daily personal care. For example, Roman bathing often relied on oils: people rubbed oil into the skin and scraped it off with a tool called a strigil. The principle behind oil cleansing still makes sense chemically—“like dissolves like”—so oil removes oily residues like makeup and sunscreen effectively.
Medical advocates in the 19th century demonstrated soap’s life-saving power before germ theory was fully understood. Hungarian physician Ignaz Semmelweis and Florence Nightingale promoted handwashing in clinical settings in the mid-1800s. Later, Louis Pasteur and Joseph Lister developed the scientific understanding that microbes cause disease. Yet it took until the late 20th century for public health authorities to formalize routine handwashing recommendations.
Materials used to make early soaps
Early soaps generally used animal fats—tallow, lanolin, lard—or even goat milk combined with alkaline ash. Unrefined animal fats produced soft, dingy, and odorous soaps. Rendering or purifying those fats improved appearance and scent, but rendering required long, smelly processing. Because both soap and candles used rendered fat, soapmakers and chandlers were often the same people. My family’s first soapmaker, Emanuel Heilbronner of Laupheim, Germany, made both candles and soap in 1858.
History of vegetable oil-based soap
The earliest widely recognized vegetable-oil soaps came from Aleppo (modern Syria) beginning in the 8th century AD. Artisans there used abundant local olive oil and bay laurel oil to make a gentler soap than animal-fat variants. Aleppo soap is traditionally green inside from laurel oil and develops an ivory crust as it cures, which helps authenticate traditional bars.
Making Aleppo soap was an involved process. After cooking the oils with alkali for days, the mixture was poured into a frame on the floor to cool, then cut into bars and stacked in tall, open towers to cure for months. This curing produced hard, mild, and long-lasting bars. Nearby Nablus (in historic Palestine) produced a simpler olive-oil soap that followed similar traditions. Some traditional soapmakers in these regions still use ancestral methods today.
The advent of Castile soap
Castile soap takes its name from the historic Crown of Castile in Spain. How soapmaking arrived in Spain is not definitively recorded. The Phoenicians, Romans, and later the Moors all had contact with the Iberian Peninsula and practiced cleanliness and trade, so the technology likely spread through various routes. Arabic influence remains visible in Spanish terms related to soap and alkali, such as almona (soapworks) and alcali.
What distinguished Castile soap was not just olive oil but also the high-quality alkali used in the region. Until the late 18th century, alkali was made by drying and roasting certain salt-tolerant plants to produce soda ash. Spanish soda ash, called barrilla, came from native Salsola plants and produced a highly soluble alkali prized across Europe. Spain protected that resource tightly—exporting seeds was restricted—and the barrilla trade was economically important.
Spanish soapmakers also used a salting-out, or graining, technique: adding salt water caused soap to separate and float, leaving behind lye and impurities. Soap made with high-quality alkali and this salting process was harder, longer-lasting, milder in pH, and distinctly white—characteristics that made Castile soap a luxury export.
The decline of Castile soap
Several developments reduced Castile soap’s dominance. In 1791 Nicolas Leblanc developed an industrial method to produce alkali from common salt, making alkali more widely and cheaply available. That innovation, scaled in industrial centers, diminished reliance on Spanish barrilla.
Castile soap also lacked the legal protections enjoyed by French Marseilles soap, which was regulated by royal decree to be made in Marseille with a high olive-oil content. Without such protections, the Castile name became vulnerable to imitation and adulteration. Political and religious tensions also played a role: English markets began calling Spanish imports “Catholic soap,” discouraging Protestant consumers and encouraging national soap production. Combined with Spain’s declining geopolitical power, these factors reduced Castile’s market share and led many traditional soapworks to close.
My family’s history making Castile soap
Although Castile soap had been imported as a luxury to the United States for centuries, my grandfather Emanuel Bronner was the first in his family to produce it in America, beginning in the 1940s. He came from a line of German soapmakers who had worked with animal fats and lanolin; once in America he focused on vegetable-based Castile soap and promoted its mildness and versatility.
Emanuel marketed his liquid Castile soap with ever-more inventive labels listing multiple uses—“9-in-1” became “13-in-1” and later “18-in-1.” He valued vegetable oils for their lower environmental impact compared with rendered animal fats or petroleum derivatives and emphasized biodegradability and natural ingredients well before those trends were mainstream.
Another innovation was offering soap as a liquid, achieved by using potassium hydroxide rather than sodium hydroxide. Liquid soap allowed larger containers of oil-based formulas and gave him more label space to share his messages—something he considered important. Years later, the company also produced bar soaps.
Castile soap today
Castile soap is enjoying renewed popularity as consumers seek plant-based, biodegradable cleansers that are gentle yet effective. Today, “Castile” generally indicates a vegetable-oil soap, but the term is not legally protected. Shoppers should read ingredient lists to ensure a product labeled Castile doesn’t include animal fats, petroleum derivatives, synthetic detergents, or artificial foaming agents.
Ultimately, soap’s basic chemistry has endured for millennia. That continuity is a reminder of how basic human needs—cleanliness, health, and resourcefulness—connect us to generations past who used the best knowledge available to care for themselves and their communities.
Further reading
- Bronner Haus Museum: My Family’s Story of Restoration & Reconnection
- Five Generations of Soapmakers Built Dr. Bronner’s
- Dr. Bronner’s Factory – Bar Soap Tour with Lisa Bronner!