Borax is better known on the workbench than in the darkroom, but it has been part of black-and-white photographic chemistry for a very long time. In film developers it is the quiet ingredient that sets the pace: a mild alkali that switches the developing agents on gently, holds the solution at a steady working level and helps produce fine grain. It also turns up in some toners and in the buffered formulas that people mix themselves from published recipes. This guide explains what borax actually does in photographic solutions, how to mix and store it accurately, and how to handle it safely in a professional or studio darkroom in the UK.
Why developers need an alkali
A black-and-white developer has four basic jobs, and each is usually handled by a different ingredient:
- Developing agent(s): the chemicals that reduce exposed silver halide to metallic silver, such as metol, hydroquinone or phenidone.
- Preservative: usually sodium sulphite, which slows the oxidation of the developing agents by air.
- Accelerator (alkali): most developing agents are only weakly active in neutral solution and become far more energetic as alkalinity rises.
- Restrainer: often potassium bromide, which holds back fog on unexposed areas.
Borax belongs in the third group. The choice of alkali has a large effect on how a developer behaves. Strong alkalis such as sodium hydroxide produce fast, contrasty, high-energy developers. Sodium carbonate sits in the middle and is the usual choice for paper developers. Borax is one of the mildest options, which is exactly why it is used where slow, controlled development is wanted.
What borax does in a film developer
A gentle accelerator
Dissolved in water, borax gives a mildly alkaline solution. That is enough to activate developing agents such as metol, but not enough to drive them hard. The result is a developer that works steadily rather than aggressively, which gives the photographer more control over development time and contrast, and is more forgiving of small timing errors.
A buffer that keeps results consistent
Borax is a buffer: its solutions resist changes in pH when small amounts of acid or alkali are added. This is why it is also a standard ingredient in laboratory buffer solutions. In a developer, the chemistry of development and the gradual oxidation of the solution would otherwise shift the pH as the developer is used. A borate buffer damps that drift, so the tenth film through a batch behaves more like the first. Some published formulas pair borax with boric acid to set the working pH a little lower and hold it there even more firmly.
Fine grain and smooth tonality
Classic fine-grain film developers combine metol and hydroquinone with a high proportion of sodium sulphite and a small amount of borax. The low alkalinity, together with the solvent action of the sulphite, keeps grain fine and gradation smooth. Many photographers regard this type of metol-hydroquinone-borax developer as the reference point against which other film developers are judged. Changing the amount of borax changes the activity of the developer, which is why formula variations exist for different films and contrast needs, and why it matters to weigh it accurately.
Other places borax appears in photographic chemistry
- Toners: some traditional gold toners for printing-out and salted-paper processes use borax as the alkali to set the working conditions of the toning bath.
- Buffered and two-bath developers: borax, sometimes with boric acid, is used to adjust or stabilise alkalinity in specialist film developers.
- Replenishers: for deep-tank processing, replenisher formulas may contain a higher proportion of alkali than the working developer to make up for what is consumed.
In every case the principle is the same: borax is there to control alkalinity precisely. Always work from a published formula from a reputable source and follow it exactly, including the order in which the chemicals are dissolved.
Which borax to use for photographic solutions
Most photographic formulas that call for “borax” mean disodium tetraborate decahydrate, Na2B4O7ยท10H2O, the standard commercial form. The pentahydrate and anhydrous forms contain less water, so the same weight carries more borate. Substituting one for another without adjusting the quantity will make the developer more active than the formula intends. Our guide to the borax label, SDS and spec sheet shows how to identify each hydrate by its CAS number and how to convert between them.
Purity matters too. Impurities such as iron, chloride or sulphate can affect fog and keeping properties. Look for a stated assay and batch number. Our Borax Decahydrate Powder 99.9% is supplied with a Safety Data Sheet and comes in 900g and 3kg packs; you can compare both in the borax powder range. Note that a borax substitute (a blend of washing soda and baking soda) is not borax and must never be used in a photographic formula that calls for borax.
Mixing borax into a developer: practical points
- Dissolve it fully. Borax dissolves slowly in cold water and much more readily in warm water. Use the water temperature stated in your formula and stir until no crystals remain before adding the next chemical. Our borax solubility chart explains how temperature affects dissolving.
- Follow the order of mixing. Developer formulas list ingredients in a specific order for good reasons, for example so that the preservative is present before the developing agents meet the alkali. Do not change it.
- Weigh accurately. Because borax is used in small quantities, a small weighing error is a large percentage error. Use dedicated scales that are never used for food.
- Use clean water. Many darkroom workers use distilled or deionised water for stock solutions to avoid variation from tap water minerals.
- Keep equipment separate. Use graduates, jugs, stirring rods and storage bottles kept only for photographic chemistry. Never use kitchen utensils or food and drink containers.
Storing borax and mixed solutions in the darkroom
Darkrooms are often humid, which is hard on dry chemicals. Keep the borax pack tightly sealed, off wet benches and away from sinks and washing trays, or it will cake. Our article on how to store borax powder covers caking, containers and secure storage in detail.
Mixed developer should be stored in full, tightly closed, clearly labelled bottles to limit contact with air. Label each bottle with the solution name, the date it was mixed and the hazard warnings of its ingredients. Never store photographic solutions in drinks bottles, and keep them where children and visitors cannot reach them.
Disposal of used darkroom solutions
Spent photographic solutions are not simply waste water. Used fixer in particular contains dissolved silver, and developers contain several hazardous ingredients besides borax. Follow section 13 of each Safety Data Sheet, and the rules of your local water company or waste contractor. Businesses discharging process waste may need permission from their water company, so check before pouring anything away. Do not dispose of borax powder or solutions in garden soil or surface drains.
Safety summary
Borax (disodium tetraborate) is classified in Great Britain as toxic for reproduction, category 1B, with the hazard statement H360FD: may damage fertility; may damage the unborn child. We supply it for professional, industrial and laboratory use only. Other darkroom chemicals carry their own hazards, so read every Safety Data Sheet, not just the one for borax.
- Keep out of reach of children. Not for use by children or in children’s activities.
- Wear chemical-resistant gloves and eye protection when weighing powder and handling solutions.
- Do not swallow. Do not eat, drink or smoke in the darkroom, and wash your hands after handling.
- Avoid raising dust when weighing, and make sure the darkroom is properly ventilated.
- Do not use if you are pregnant or trying to conceive.
- Keep borax in its original labelled packaging, and label every bottle of mixed solution.
- Follow the product label and Safety Data Sheet, and include darkroom chemicals in your COSHH assessment.
Borax in photography: FAQ
Why is borax used in film developer?
Borax is a mild alkali and buffer. It activates developing agents such as metol gently and keeps the pH of the solution steady, which gives slow, controllable development, fine grain and consistent results from one film to the next.
Can I use sodium carbonate instead of borax in a developer?
Not as a straight swap. Sodium carbonate is a stronger alkali, so it makes a more active, more contrasty developer with coarser grain. Use the alkali the formula specifies, or a published variant designed for a different alkali.
Which type of borax do photographic formulas mean?
Unless the formula says otherwise, “borax” means disodium tetraborate decahydrate. The pentahydrate and anhydrous forms contain less water, so the quantity must be adjusted if you use them.
Can borax substitute be used in darkroom chemistry?
No. Borax substitute is a boron-free blend of washing soda and baking soda for household cleaning. It has different chemistry and will not give the results a borax formula is designed for.
How should I store borax in a darkroom?
Keep it sealed in its original labelled pack, away from sinks and wet benches so it does not cake, in a secure place out of reach of children and away from food and drink.
This guide is general information for professional users. Always follow the label and Safety Data Sheet supplied with each product and your own COSHH assessment.