What Is Forest Glass and Where It Came From
Forest glass, known in German as Waldglas, is a type of medieval glass made in northwestern and central Europe from about 1000 to 1700 AD. It was produced using wood ash and sand. These factories, called glasshouses, were mostly built in forested regions to take advantage of nearby firewood. The glass often shows greenish or yellowish hues. Early pieces were simple and of lower quality. Originally used for everyday items like vessels, it gradually found its way into church stained glass windows. This form of glass production is distinctly different than the Roman methods and those used in the Byzantine and Islamic worlds.
How Glassmaking Changed After Rome
During Roman times, northern Europe used the same glassmaking ingredients and techniques as the Romans, relying on natron, a mineral flux. After Rome fell around 450 AD, people recycled older Roman glass, and fresh glassmaking skills declined. By around 800 AD, as the Carolingian Empire expanded in northwestern Europe, demand for new glass rose. But traditional materials were often expensive and hard to get. Rulers wanted to rival or beat the quality of Byzantine and Islamic glass. That drove experimentation, which led to a new glassmaking process using different raw materials.
Glasshouses Across Europe
Archaeologists have found many medieval glasshouse sites throughout western and central Europe, particularly in Germany’s mountain areas. Most sites are not well-preserved today, since people often reused their building materials. But evidence shows that both glass raw material production and glass finishing (turning raw glass into vessels or windows) were often done at the same location.
From Raw Material to Finished Product
It helps to separate two stages of glassmaking. First is the creation of raw glass from scratch. Second is glass working - melting raw glass or cullet to shape finished items. Raw glass requires four main ingredients:
1- Former (Silica)
Silica (SiO₂) forms the basic network of glass. Ancient makers used crushed quartz. Later, they switched to sand.
2- Flux (Alkali)
Flux lowers the melting point of silica so it melts in the kiln. Ancient Mediterranean makers used sodium-rich plant ash. Romans used natron from Egypt’s Wadi El Natrun. Islamic makers returned to plant ash. In Northern Europe, glassmakers used wood ash to create potash (K₂CO₃) as a flux. Lime (CaO) also plays a role in lowering melting temperatures.
3- Stabiliser
A stabiliser helps prevent glass from dissolving in water. Lime is the most effective stabiliser. Sand may already contain lime or additives like alumina and magnesia.
4- Colorants and Opacifiers
Color in glass comes from impurities or additives. Common colorants include iron, copper, cobalt, manganese, tin, antimony, and lead. Opacity is achieved by adding agents like tin or antimony, or by trapping bubbles in the glass. Furnace temperature and the presence of oxygen determine color and opacity.
Why It Mattered
Forest glass represents a change in European technology after the fall of Rome. It shows innovation in using wood-derived flux and forest-based glasshouses. Over time, it evolved from crude household glass into materials suitable for church windows. This transition highlights how medieval Europe developed its own glassmaking traditions, distinct from southern and eastern Mediterranean practices.
Post‑Roman Changes in Glassmaking and the Rise of Alternatives
After Rome fell, political instability in the Wadi El Natrun disrupted the supply of natron. Eastern Mediterranean glassmakers switched back to using sodium-rich plant ash. They then exported glass across southern Europe via old Roman trade routes. Venetian artisans, inheriting Roman glass techniques, cornered this market. They controlled plant ash supplies and forbade their workers from moving outside Venice. This left northern Europe above the Alps without access to natron or plant ash, forcing them to innovate.
Sand or quartz and lime, needed for the base and stability of glass, were readily available almost everywhere. Northern European craftspeople experimented with wood ash, fern ash, and bracken ash to create an alkali flux. This new raw material led to a new technology, complete with its own challenges. Roman glass had been prized for its thinness, clarity, and consistent quality. In contrast, early Forest glass showed green‑brown hues, thick walls, bubbles, and inclusions. Those differences meant wood‑ash was not a simple substitution; it required reevaluating the entire production process.
Why Medieval Forest Glass Varied More Than Roman Glass
Roman-era glass was mostly Si/Na/Ca (silica, sodium, calcium) and held a consistent composition across time and region. Post‑Roman medieval glass, now Si/K/Ca (silica, potassium, calcium), varied far more. The change lies in how melting points depend on these components. In simple Si/Na/Ca batches, varying proportions didn’t affect melting temperatures much, so glass composition stayed stable. With Si/K/Ca mixes, melting temperatures changed noticeably when the ratio changed. That meant the old process - partial melting, skimming off unmelted residue, and repeating - no longer produced consistent results. Glassmakers had to find a new method.
Evidence shows that medieval workshops switched to fully melting each batch and then removing unreactive material as scum. Archaeological analysis of glass compositions and historical accounts support this idea.
Refining Forest Glass Quality After 1400
Around 1400 AD, European glassmakers sought to match the clarity of Venetian glass. They began adding calcium oxide to their potash and sand mix. They used sources like limestone, marble, or shells to provide CaO. This new recipe allowed them to lower the amount of potash needed, which reduced unwanted coloration. The result was a clearer, purer glass better suited to windows and fine vessels.
Comparative Compositions of Historical Glass
Here is a clear comparison of the main components in glass from different periods and regions, with weight percentages given for silica, soda, potash, lime, and magnesia. Ancient glasses also contained small amounts of iron oxide and alumina, plus various colorants and opacifiers.
- Egyptian (15th century BC): silica 65 %, soda 20 %, potash 2 %, lime 4 %, magnesia 4 %.
- Roman (1st century AD): silica 68 %, soda 16 %, potash 0.5 %, lime 8 %, magnesia 0.5 %.
- European (13th century AD): silica 53 %, soda 3 %, potash 17 %, lime 12 %, magnesia 7 %.
- Syrian (14th century AD): silica 70 %, soda 12 %, potash 2 %, lime 10 %, magnesia 3 %.
- Modern: silica 73 %, soda 16 %, potash 0.5 %, lime 5 %, magnesia 3 %.
Egyptian glass used plant ash and quartz as its ingredients. Roman glass relied on natron and sand. Medieval European glassmakers used wood ash mixed with sand or quartz. Syrian glass followed a plant ash and sand/quartz blend, while modern glass uses synthetic components.
Why Forest Glass Varied in Color and Clarity
Medieval European glassmakers adapted to wood-ash methods but found it hard to match the clarity and color consistency of earlier Roman glass. European sands and soils usually contain more iron and manganese. Iron in glass often resulted in a blue-green or yellow tint. Manganese introduced purple hues, which could offset iron’s effect and produce colorless glass.
When beech wood grew on lime-rich soils, such as in Kleinlutzel in Jura, the resulting wood ash had high manganese content, yielding nearly clear glass. In contrast, in clay-rich regions like Court-Chalvet in Jura, glass took on an olive green shade. Glassmakers learned to experiment and gradually moved from dark, muddy hues to clearer glass. In late 16th-century Bohemia, artisans used manganese’s decolourising effect to create glass clear enough for engraving.
The amount of carbon left behind in the wood ash also changed the furnace atmosphere and influenced the final color of the glass. Analysis of glass from York Minster shows roughly 90 percent of its color came from natural raw material chemistry, not added colorants.
Adding Color and Opacity by Design
Glassmakers could also add metal oxides to control color. Copper oxide produced green or turquoise hues, while cobalt created deep blue. Creating red glass was especially challenging, because it required suspending copper particles under precise furnace conditions with controlled oxidation and reduction. There is little evidence medieval Europeans used antimony or tin to make opacifiers, or used lead to adjust colors.
Historical Records and Glassmaking Sites
We have only two medieval European texts detailing glassmaking. In 1120, Theophilus Presbyter in Germany recorded detailed recipe instructions. In 1530, Georgius Agricola wrote about contemporary European glass production. Additional insight into medieval techniques comes from archaeological remains and modern experimental reconstructions. In Thuringia, families like the Greiners and Muellers set up glass huts in towns such as Lauscha, marking the beginning of an enduring European glassmaking tradition.
How Medieval Glassmakers Got Their Ingredients
Medieval glassmakers gathered sand from river beds. River sand was ideal since it was clean and uniform in size. They also used wood both to make ash and to fuel their furnaces. Cutting trees, transporting logs, drying them, and storing them were all labor-intensive. This process needed careful planning and coordination.
Why Wood Type Mattered
Theophilus, a medieval writer, advised using beech logs. Beech wood from lime-rich soils contains more calcium oxide, which benefits glass quality. Still, any wood’s contribution of potash and calcium oxide depended on many factors. The age and part of the tree, the soil it grew in, the climate, when it was cut, and how dry it was all affected the ash’s properties. Glassmakers had little control over these variables. That caused inconsistency in glass batches. To reduce variation, they mixed large batches of ash for a more uniform result. Since beech yields only around one percent ash, it took about 63 kilograms of wood to make one kilogram of glass using the recommended sand-to-ash ratio. Overall, each kilogram of finished glass required between 150 and 200 kilograms of wood - ash and fuel combined.
After mixing ash and sand, glassmakers heated them together without fully melting them. This process, called fritting, took place at roughly 900 degrees Celsius or 1650 degrees Fahrenheit. Theophilus said this step should last a full day and night. As the mixture changed color with rising heat, the sand and ash bonded, reducing volume and helping settle loose ash. This made it easier to transfer the material into crucibles and limited furnace openings.
The fritted mixture was then moved into crucibles within a covered furnace for the final melt. At this point, furnaces needed to reach much higher temperatures - about 1350 degrees Celsius, because potash melts at a higher temperature than natron. This change forced glassmakers to redesign their furnaces and develop ceramics that could withstand such intense heat. Regular clay could not handle the new temperature without reacting with the glass.
Once molten, this glass could be blown into vessel shapes or blown into cylinders. Cylinders would be cut and flattened into window panes. After shaping, the glass underwent annealing. This slow cooling process eased internal pressures that form during heating, preventing cracking or warping.
Medieval Forest Glasshouses: Inside the Workshops
The only visual record of a forest glasshouse from the Middle Ages is a Bohemian illustration known as the Mandeville Miniature, drawn around 1380. It shows a single building built of wood, enclosing several ovens tuned to different temperatures. Nearby, workers mixed sand and ash at a pit. They then carried the mix in pans to one oven for fritting, heating it up to 1100 °C. Once fritted, the material was moved to another oven, where crucibles melted the glass at around 1400 °C. When molten, glassworkers blew the liquid into shapes and then moved the pieces into a cooling oven. This cooling stage, called annealing, returned the glass to a safer temperature slowly to avoid cracks. Above the furnace, wood was probably stored and dried inside the building.
Later finds include the ruins of a similar glasshouse from the late 15th century in Eichsfeld, Germany. By the 17th century, archaeologists also discovered a design called the butterfly furnace. These stone-built furnaces used crucibles made from specially imported, high-heat clay. This layout differed from southern Europe’s beehive furnaces, where the annealing oven sat atop the main chamber, and from the eastern Islamic designs.
Fuel, Size, and Scale
Glassmakers aimed to balance wood usage, production output, and labor efficiency. As techniques improved, larger workshops kept furnaces burning nearly nonstop. On average, a big forest glasshouse could burn about 67 tonnes of wood every week and operate about 40 weeks a year.
Because glassmaking consumed so much wood, glasshouses were set up deep in forests and managed carefully. Coppicing and pollarding methods ensured a steady supply of wood and produced pieces the right size for burning. Still, once an area’s woodland was worn out, the glasshouse often moved. Glassmakers also had to compete for wood with mines, homes, and other industries. In 16th century England, use of wood for glassmaking was even restricted. Many glasshouses belonged to churches or were set up in church forests since one key use of forest glass was for church windows.
“The Cold Heart” Fairy Tale
“The Cold Heart,” originally titled Das kalte Herz in German, is a captivating fairy tale penned by Wilhelm Hauff. It was first published in 1827 within a collection of stories framed by a narrative set in The Spessart Inn. Hauff’s tale has inspired German film adaptations, notably in 1924 and again in 1950, each sharing the story’s original title.
Who Is Peter Marmot?
Peter Marmot, also known as Coal‑marmot Peter, inherits his late father’s charcoal business. He finds the work grim, exhausting, and low-paying. Peter craves wealth and social standing far beyond what his dirty trade can provide.
The Enchanting Glass‑Imp
Rumor spreads about a forest spirit called the little glass‑imp or Schatzhauser, believed to dwell in the Black Forest. It is said this imp grants three wishes to anyone born on a Sunday between 11 am and 2 pm, as Peter was. To summon him, one must recite a specific poem. During his quest, Peter encounters another forest spirit named Dutch‑Mike, a fearsome sorcerer known to haunt the woods on stormy nights. Peter manages to escape from Dutch‑Mike.
The Summoning Poem
Peter recites the lines:
“Schatzhauser in the green firwood, thou art many hundreds of years old. Yours is the land where the firs stand, thou shalt only be seen by Sunday’s children.”
This summons the glass‑imp, who offers him three wishes with a warning: if the first two are foolish, the third will be withheld. Peter wastes no time and makes two impulsive demands.
Peter’s First Two Wishes
Peter’s first wish is to dance better than the local “dance floor king” and to match his rival, Ezekiel, in wealth. The second wish is more sensible: he asks for a large glass factory and enough money to run it. The imp warns Peter: he should have asked for wisdom to manage it. He withholds the third wish until Peter proves himself worthy.
Success, Mismanagement, and Downfall
At first, Peter seems lucky. He acquires a magnificent glass factory in the Black Forest, becomes a superb dancer, and gambles successfully, matching Ezekiel’s riches. He rises to local fame. But his ignorance in running the factory becomes painfully clear once he abandons it for gambling. He racks up debts, and his finances suffer.
Then his win streak flips. He gambles with Ezekiel again and suddenly loses everything - not just his money, but even Ezekiel’s. He is soon thrown out of the tavern. The next morning, a magistrate arrives to confiscate his glass factory.
The Fateful Deal with Dutch‑Mike
In despair, Peter returns to the forest to seek help from Dutch‑Mike, a spirit tied to darker forces. Mike offers Peter an even greater deal than the glass‑imp, but demands something terrible in return: Peter’s heart. Peter, who sees his heart and emotions only as liabilities, is ready to give it up. Dutch‑Mike replaces Peter’s heart with a cold stone and grants him 100,000 thalers per day, with the option to return whenever he wants more money.
A Harrowing New Life
With his new fortune secured and his senses numbed by the stone heart, Peter embarks on a whirlwind journey around the world. What happens next is a chilling reflection on greed, ambition, and the loss of humanity.
The Glass‑Imp and the Stone Heart
Soon Peter realizes that his new stone heart brings no joy. He cannot laugh or cry. He cannot love. Nothing seems beautiful. He no longer feels empathy. Confused and empty he returns to the Black Forest. He seeks out Dutch‑Mike, hoping to get his heart back. Dutch‑Mike refuses. He tells Peter only death will return his feelings. Dutch‑Mike shows him a collection of other hearts. He reveals that many prominent Black Forest figures lost their hearts for money. His old friend Ezekiel is among them. Dutch‑Mike gives Peter more money and suggests he find work and a wife to relieve his boredom.
Peter follows this advice. He builds a mansion and works as a merchant. He also collects debts at very high interest. He gains a reputation for meanness and drives beggars away. Even his sick mother receives only a small allowance while he keeps her at a distance. Next he marries Lisbeth the beautiful. At first their marriage seems grand but day after day Lisbeth grows unhappy. Peter is harsh and stingy. He tells Lisbeth not to help the poor. Their wealth adds to her misery. Soon she is seen as even harsher than Peter.
One day an old man arrives at the mansion and asks for drink. Lisbeth offers him wine and bread when she thinks no one is watching. The man thanks her and says her good heart will be rewarded. Just then Peter returns. He flies into a rage and strikes Lisbeth with a whip handle. She falls dead. Seeing her dead body Peter is filled with regret. The old man then reveals himself as the glass‑imp. He tells Peter that Lisbeth was the most beautiful flower of the Black Forest. Enraged, Peter blames the imp. The imp transforms into a monster out of his own anger.
For Lisbeth’s sake the imp gives Peter eight days to reflect on his life. Peter cannot sleep. Voices in his head tell him to get a warmer heart. In public he lies about Lisbeth, saying she has gone traveling. His guilt makes him meditate on death. Finally on the eighth day he enters the woods and calls on the little glass‑imp. He still has one wish left. He asks to regain his heart. The glass‑imp tells him he cannot help. The contract was with Dutch‑Mike. Instead the imp shows Peter how to trick Mike.
That night Peter meets Dutch‑Mike again and claims betrayal. Peter says the stone heart in his chest is not real. Dutch‑Mike insists on testing this claim. He removes the stone and restores Peter’s real heart. Peter then shows a glass cross given by the glass‑imp. This scares Dutch‑Mike away. Peter rushes to the glass‑imp. Now truly filled with regret he reunites with his mother and witnesses Lisbeth come back to life.
On advice from the imp, Peter returns to honest work as a charcoal burner. He finds joy without wealth. He rebuilds his reputation through hard work. When his son is born the glass‑imp gives him four rolls of thalers as a gift. The imp becomes the godparent of Peter’s son.
Wilhelm Hauff based the second half of this fairy tale on Washington Irving’s short story “The Devil and Tom Walker.” He reshaped it into a story that feels like a Black Forest legend. The tale also reflects the Romantic Era. During that period writers often used fairy tales to explore human emotion. Hauff uses the idea of the glass‑imp to explore the Romantic longing for happiness. The story includes mysterious and eerie elements as well as folk traditions. The main idea of Romanticism - longing or sehnsucht - is embodied by Peter. He receives three wishes. His longing leads not to happiness but to destruction. His riches slip into misery. His longing becomes greed and bitterness. Yet he does not give up. In classic Romantic fashion he finally finds true happiness with love and honest work.
