What If I Travel Back In Time

A survival-technology knowledge base: how to recreate useful materials, machines, and practices using what is obtainable in a pre-industrial era. Every guide assumes you arrive with knowledge only — no carried tools, chemicals, or electricity — but inside a settled society whose ordinary tools, materials, and skills can be borrowed, earned, or commissioned.

New here? Read The First Year. The roadmap below is a dependency-informed priority map, not a literal build order; that page turns it into a practical sequence.

How this repo is organized

• fundamentals/ — the prerequisites almost everything depends on. Read these first.
• materials/ — physical things you make: substances, foods, and goods.
• technology/ — machines, instruments, and infrastructure.
• knowledge/ — concept-led practices whose main advantage is transferable understanding, though applying them still needs people and materials.

Each guide opens with a Difficulty and Prerequisites line and a Why it matters section, then works through the process, and closes with two things that are the point of the whole exercise:

• a Failure modes table — symptom, cause, fix — because diagnosis is the actual advantage a modern mind brings;
• Era notes — when this was really invented, and what specifically you'd be introducing rather than reinventing.

Scope, evidence, and limits

This is a historical thought experiment and an orientation guide, not a set of certified engineering drawings, food-processing schedules, or medical instructions. Location and era change almost every recommendation: climate, geology, crops, law, labor, existing craft knowledge, and access to tools must be checked before choosing a project.

Exact-looking numbers are not automatically exact in the field. Raw-material purity, altitude, humidity, vessel geometry, and measurement error all matter. Start with small trials, keep records, and do not scale a hazardous process on the strength of one paragraph. Safety-critical and quantitative corrections are tracked in Sources and limits; proposed changes should follow the contribution guide.

The golden rules of time-traveler technology

1. Chemistry and temperature work together. Pure silica melts near 1710°C; a correctly calculated silica–alkali–lime batch can begin melting much lower, though fining and working still demand a sustained high-temperature furnace.
2. Charcoal is a key furnace fuel — but wood-to-charcoal yield varies with species, moisture, kiln, and operator, so measure local yield before scaling. Fuel supply can constrain every furnace.
3. Everything starts with fire, clay, and ash. Kilns come from clay. Lye, potash, and glass flux come from ash. Lime comes from fired limestone/shells.
4. Fermentation is powerful controlled biology. Food safety depends on the right organism, substrate, salt/acidity, temperature, sanitation, and endpoint. Some processes tolerate managed spontaneous fermentation; sake and soy sauce require authenticated food-grade koji rather than captured wild mold.
5. Separation starts with washing, settling, sieving, and recrystallizing. Those operations can enrich or clean a material, but appearance is not proof of identity, purity, dose, or absence of toxins; high-stakes uses need analysis.
6. Atmosphere is a control, not an accident. Oxidizing conditions support ore roasting, refining, and some ceramic colors but can also oxidize exposed metal; reducing conditions are required for some reduction stages and ceramic effects. The right atmosphere depends on the material and process stage.
7. Labor is the real constraint, not knowledge. Almost every process here is possible by hand and uneconomic by hand. A waterwheel is what turns "we could make that" into "we make that by the ton."
8. Some references can be bootstrapped by comparison. Alternating three plates with marking and selective scraping can generate flatness without a master plane. Length, pitch, angle, and absolute scale still need standards, calibration, and uncertainty checks; a leadscrew copies its own systematic error unless you measure and compensate it. See precision and toolmaking.
9. Diagnose with the local experts. Craftspeople bring generations of empirical knowledge; explicit mechanisms, controlled trials, and written records make that knowledge easier to test and extend. That's what the failure-mode tables are for.
10. Write it down and print it. Knowledge that lives in one head dies with that head.

What actually matters most

Every page here argues for its own importance. This list is the tiebreak, and the individual guides defer to it. Ranked by lives and welfare per unit of effort, not by how impressive the technology is:

1. Germ theory, demonstrated rather than argued — microorganisms cause many infectious diseases, but routes differ. Hand hygiene, safe water, ventilation, isolation, vaccination, and disinfection must match the organism and exposure.
2. Oral rehydration therapy — when packaged ORS is unavailable, six level teaspoons of sugar and half a level teaspoon of salt in one litre of safe water is an emergency substitute. Severe dehydration still needs urgent skilled care and often intravenous fluid.
3. Compatible legume–rhizobia systems fix nitrogen from the air. The net benefit depends on inoculation, soil, crop, harvest, and how residues return to the field; verify nodulation and yield locally.
4. Smallpox isolation and verified vaccination — historically transformative when a genuine vaccine source and safe containment existed. Improvised variolation can kill the recipient and start an outbreak.
5. Separate the sewage from the drinking water, and boil what you drink.
6. Writing, then cheap paper, then printing — the chain that makes everything else outlive you.
7. Winter fodder crops, which break the autumn-slaughter cycle and close the manure loop that feeds rule 3.
8. For small swings, a pendulum's period is set mainly by its length and local gravity. At standard gravity, about 0.994 m gives a one-second half-swing; temperature, amplitude, and the escapement still matter.
9. Spectacles. Where clear glass, repeatable grinding, fitting skill, and a literate market exist, reading lenses can extend many people's useful working lives.
10. Water power, which can multiply suitable repeatable work where measured flow, head, seasonal reliability, rights, and machinery justify a mill.

Entry-level prevention and record-keeping can use existing household or farm capabilities. Vaccination, severe-illness treatment, constructed water systems, and any scaled intervention still require authenticated materials, reliable measurements, cooperation, and specialist skill.

Where to start — a dependency-informed roadmap

The tiers show priority and broad capability, not a strict topological sort. Entry-level versions often use borrowed tools or existing local crafts, while a guide's prerequisite line describes what its full process needs.

Tier 0 — before anything else

1. Clay and earth building — shelter, kilns, and furnace linings, with no fuel required
2. Fire, fuel, and high temperatures — learn controlled fuel, airflow, insulation, atmosphere, and carbon-monoxide safety
3. Pottery and ceramics — you need containers before you can make anything in them
4. Woodworking and joinery — every machine below is made of wood
5. Woodland and fuel supply — cut wood so it grows back, before you scale anything up

Tier 1 — cheap, fast, immediately valuable

1. Medicine and public health — prevention principles first; clinical work needs trained practitioners
2. Water supply and sanitation — source protection and boiling first; constructed works need more materials
3. Mathematics and record-keeping — low-material methods that make every other page cumulative
4. Writing and literacy — start records now; cheap paper and printing come later
5. Agriculture and crop rotation — the food surplus is the budget for all of the above
6. Salt — preservation and trade income where a viable source exists
7. Food preservation — turns a harvest glut into a longer-lasting supply and cooking and nutrition — daily meals, fuel economy, and the deficiency diseases plus extracts and by-products — useful materials from what other processes throw away
8. Horticulture and propagation — the garden and the orchard, and grafting, without which good fruit cannot be reproduced at all
9. Mushroom cultivation — turns straw, sawdust, and coppice poles into food, and is safer than foraging
10. Hunting, fishing, and wild food — the buffer when the harvest fails, and where hides come from
11. Grain, bread, and beer — most of the calories most people eat, and small beer as a boiled alternative to suspect water
12. Dairy and cheese — concentrates a daily perishable into something that stores and trades
13. Oils and fats — feeds soap, ink, leather, lamps, and lubrication
14. Basic chemicals — lime, lye, potash, vinegar
15. Soap — a high health return once alkali and fat are available
16. Lime, mortar, and concrete — same kiln as the lime above, plus buildings and soil amendment

Tier 2 — needs a real furnace

1. Mining and prospecting — surface prospecting and assaying first; deep mining needs later-tier tools and power
2. Copper and bronze — learn controlled crucibles, moulds, ores, fumes, and furnace atmosphere before attempting iron
3. Glass — and then lenses, which is where it gets interesting
4. Iron and steel — hardest and most valuable; don't start here
5. Silver — money, but read the lead warnings first

Tier 3 — multiply your labor

1. Mechanical power — measured water or wind resources can multiply suitable repetitive work, with maintenance and guarding
2. Precision and toolmaking — files, flats, screws; the gate on clocks, instruments, and engines
3. Structures and masonry — arches, vaults, foundations, and the weir a mill depends on
4. Textiles and fiber — the biggest labor sink in any pre-modern economy
5. Leather and tanning — bellows, harness, drive belts, boots
6. Clothing, sewing, and footwear — the step that turns cloth and leather into something wearable
7. Animal husbandry — draft power, manure, and the winter fodder constraint
8. Land transport — wheels, carts, roads, bridges; drainage and gradient matter more than surface
9. Optics and lenses — spectacles, then microscopes, then everything they show you
10. Navigation and timekeeping — where you are, when it is, how much you have
11. Boats and shipbuilding — water transport is 10–20× cheaper than land

Tier 3.5 — holding together what you've built

These are organizational rather than material, and they are what historically decided whether anything built above outlasted its founder.

1. Money, credit, and public finance — how specialization gets paid for, and why cheap borrowing outperformed richer treasuries
2. Statecraft: law, records, administration — census, written law, audit, and controlling agents you cannot watch
3. Communication at a distance — relays, semaphore, telegraph; the practical limit on how large an organization can get
4. Fortification and logistics — supply arithmetic, camp sanitation, and defensive geometry

Tier 4 — long timescales, high value

1. Paper, then printing — scale the writing and records begun in Tier 1
2. Wine and vinegar — months
3. Dyes, pigments, and ink — where the trade money is
4. Sake and soy sauce — related fermentation principles, but product-specific cultures, ingredients, and controls
5. Whiskey and spirits — concentrated alcohol travels, keeps, and sells
6. Sugar — needs a climate and a mill
7. Saltpeter — 1–2 years of waiting per batch
8. Sulfur — geography-dependent; the gateway to strong acids

Tier 5 — the far end, and honest limits

1. Electricity — the first battery is easy and the gap is startling; generation and lighting are not realistically in reach
2. Steam and heat engines — achievable only after precision boring, and water power beats it wherever a stream exists
3. Penicillin — an honest account of why "grow mold, use mold" fails

Index

Fundamentals

• Fire, fuel, and reaching high temperatures — charcoal, tar, bellows and airflow, furnace building, the temperature ladder, oxidizing vs reducing
• Basic chemicals from scratch — lime, lye, potash/soda ash, causticizing, vinegar, ethanol, distillation

Materials

• Clay and earth building — finding and testing clay, cob, adobe, rammed earth
• Pottery and ceramics — tempering, kilns, glazes, refractories
• Salt — solar evaporation, brine boiling, rock salt
• Glass — sand, flux, fining, decolorizing, lead/flint glass
• Paper — pulp from bark/rags, mould-and-deckle sheets
• Oils and fats — rendering, oilseed pressing, boiled linseed oil, lighting
• Soap — lye, saponification, proportions, salting out
• Lime, mortar, cement, and concrete — lime burning, pozzolans, Roman concrete
• Copper, bronze, and brass — ore, smelting, alloying, casting, antimony
• Iron and steel — bloomery smelting, carburizing, quenching and tempering
• Silver — finding galena, smelting lead, cupellation
• Textiles and fiber — retting, spinning, weaving, fulling, rope
• Leather and tanning — liming, bark tanning, brain tanning, parchment
• Clothing, sewing, and footwear — needles and stitches, cutting and fitting, turnshoes and lasts, fur and cold-weather layering
• Food preservation — drying, salting, smoking, fermenting, cold storage
• Cooking and nutrition — cooking methods, fuel economy, food safety without refrigeration, deficiency diseases
• Extracts and by-products — starch, pectin, gelatin and isinglass, cream of tartar, essential oils, saponins, fungal materials, gourds
• Grain: milling, bread, and beer — millstone dressing, sourdough, malting, mashing, hops
• Dairy: butter, cheese, and yoghurt — rennet, curd cutting, pressing, ageing
• Wine — controlled fermentation, measurement, and keeping it from turning to vinegar
• Sake — koji, multiple parallel fermentation, staged mash
• Soy sauce — koji culture, brine fermentation
• Whiskey and spirits — malting, mashing, pot stills, cuts, barrel aging
• Sugar — cane pressing, lime clarification, crystallization
• Dyes, pigments, and ink — mordants, indigo vats, iron gall and carbon ink
• Sulfur — volcanic deposits, pyrite roasting, refining
• Saltpeter — niter beds, nitric acid, curing, fertilizer
• Penicillin — why it's out of reach, and what to do instead

Technology

• Woodland and fuel supply — coppicing, rotations, yields per hectare, seasoning
• Woodworking and joinery — grain, seasoning, joints, the lathe, coopering
• Mechanical power — waterwheels, windmills, gearing, cams
• Precision and toolmaking — three-plate flats, files, screw threads, boring, measurement
• Mining and prospecting — reading the landscape, assaying, shafts, drainage, ventilation
• Water supply and sanitation — wells, cisterns, filtration, aqueducts, latrines
• Optics and lenses — grinding and polishing, spectacles, microscopes, telescopes
• Navigation and timekeeping — compass, latitude, longitude, pendulum clocks, standards
• Boats and shipbuilding — hull construction, rigs, caulking, keeping the crew alive
• Land transport — wheelwrighting, dishing, roads, drainage, arch bridges
• Structures and masonry — arches, vaults, domes, buttresses, foundations, dams
• Printing and movable type — punches, matrices, the type mould, presses, ink
• Communication at a distance — relay posts, beacons, optical semaphore, telegraph, ciphers
• Electricity — voltaic pile, electrolysis, electroplating, electromagnets, lightning rods
• Steam and heat engines — atmospheric engines, the separate condenser, boiler safety

Knowledge

• Medicine, antiseptics, and public health — germ theory, clean water, oral rehydration, vaccination, deficiency diseases
• Agriculture, soil, and crop rotation — nitrogen fixation, the four-field rotation, soil, breeding, better tools
• Horticulture: gardens, orchards, and propagation — grafting and the cambium rule, cuttings and layering, seed saving, hotbeds, orchard siting and pruning
• Mushroom cultivation — spawn and culture keeping, log and bed methods, fruiting triggers, contamination control
• Hunting, fishing, and wild food — trap principles, weirs and nets, foraging safety, and not exhausting the resource
• Animal husbandry and veterinary basics — choosing stock, winter fodder arithmetic, selective breeding, disease, bees
• Writing and literacy — designing a script, cheap writing surfaces, punctuation and indexes, teaching it
• Mathematics, measurement, and record-keeping — positional notation, practical geometry, logarithms, double-entry, evidence
• Money, credit, and public finance — coinage and debasement, credit instruments, banking, taxation, borrowing
• Statecraft: law, records, and administration — census and survey, written law, controlling officials, relay posts, standards
• Fortification, logistics, and military organization — supply arithmetic, camp sanitation, earthworks, bastions, sieges

Plan

• The First Year — everything above, sequenced by time rather than by topic, including the risks to you personally and what not to attempt

A note on the dangerous parts

Several guides here involve genuinely hazardous work: molten metal, caustic alkali, lead, mercury, sulfur dioxide, strong acids, distillation, well shafts, and mine workings. The safety sections are not boilerplate — they describe the specific things that actually killed and crippled the people who did this work historically, often over years without their realizing why. Lead and mercury can accumulate or cause lasting injury; preventing exposure is far more reliable than trying to reverse damage later.

Three warnings are worth repeating here because the failure mode is invisible:

• Never put lead in contact with food, drink, or a still. Not glaze, not pans, not solder, not pipes.
• Sealing low-acid food anaerobically is not preservation — it can create a botulism hazard. Use a current, validated recipe and the exact required pressure/temperature, time, container, and altitude adjustment, or choose a separately validated acidification, salting, fermentation, or drying process.
• A flame that stays lit does not prove the air is safe. In a well or a mine, carbon monoxide kills without touching the flame.

The first-year, medicine, and penicillin pages are historical thought experiments about the value of modern understanding in a pre-modern setting. They are not medical advice and are no substitute for actual healthcare.

Project notes

• Sources and limits — authoritative references for the safety-critical claims and known uncertainties corrected during review
• Contributing — evidence, safety, style, and validation rules
• Generated material studies — the prompt record and disclosure for the material-guide visual collection
• scripts/check-docs.ps1 — dependency-free link, fragment, guide-schema, and image validation, run locally and by .github/workflows/docs.yml

No project license is currently declared. The repository owner should choose and add the intended documentation/code license before inviting reuse; until then, default copyright rules apply. See GitHub's guidance on licensing a repository.