About This Calculator
Paraloid B-72 (Acryloid B-72) is the gold-standard resin used by museum preparators and avocational fossil collectors worldwide. Mixing it correctly matters: too dilute and the consolidant doesn't penetrate or set; too concentrated and it coats the surface before soaking in, or becomes too viscous for fine work.
Most published guides describe concentrations in words — "2 to 10% for consolidation" — but leave you to figure out the maths yourself. This tool does the arithmetic instantly and supports three real-world scenarios:
- Fresh mix from pellets — when you're starting with dry B-72 pellets and need to know exactly how many grams to weigh out.
- Stock dilution — when you keep a concentrated stock (e.g. 15%) and want to make a specific working concentration (e.g. 5%).
- Evaporation top-up — when a jar left open has thickened up and you need to restore a working concentration by adding solvent.
How to Use This Calculator
Fresh Mix (from pellets)
- Select a preset concentration or enter your own target % w/vol.
- Enter the total volume of solution you want to end up with (in ml).
- Choose your solvent (acetone is the most common choice).
- Read off: grams of B-72 to weigh and solvent to add.
- Suspend the weighed pellets in cheesecloth, place in the jar, and pour in slightly less than the full solvent volume. Once dissolved, top up to the marked line.
Stock Dilution
- Enter your stock solution's concentration (e.g. 15%).
- Enter the working concentration you need (e.g. 5%).
- Enter the final volume of working solution you want.
- The result tells you how much stock to measure out and how much fresh solvent to add.
Evaporation Top-up
- Estimate (or measure) your jar's current volume.
- Enter your best estimate of the current concentration.
- Enter the concentration you want to restore to.
- Add the calculated volume of fresh solvent and stir gently.
Formula & Method
Fresh mix (w/vol): grams of B-72 = (target % ÷ 100) × final volume (ml). Add solvent to make up to that final volume mark.
Dilution: C₁V₁ = C₂V₂ → V₁ (stock to use) = (C₂ × V₂) ÷ C₁. Solvent to add = V₂ − V₁.
Top-up: grams of B-72 in jar = (current % ÷ 100) × current volume. Required final volume = grams ÷ (target % ÷ 100). Solvent to add = required final volume − current volume.
All calculations follow the w/vol (weight-by-volume) convention — grams of resin per 100 ml of final solution — which is the standard used by AMNH, ZOIC PalaeoTech, and Davidson & Brown (2012).
Frequently Asked Questions
What concentration should I use to consolidate a fragile fossil bone?
The AMNH recommends 2–10% w/vol in acetone or ethanol for consolidation. Start thin (2–5%) so the resin penetrates into the fossil rather than forming a surface skin. Multiple thin coats work better than one thick coat. You can increase concentration for subsequent coats once the surface is stable.
What is the difference between w/vol and w/w concentrations?
A 10% w/vol solution contains 10 g of B-72 dissolved up to a final volume of 100 ml. A 10% w/w solution contains 10 g of B-72 mixed with 90 g of solvent. Since acetone has a density of ~0.79 g/ml, these are not the same. Most published fossil prep guidance (including AMNH) uses w/vol. This calculator uses w/vol. If you work w/w, the difference at low concentrations is minor but grows at higher concentrations.
Which solvent is best for Paraloid B-72?
Acetone is the most widely recommended: B-72 dissolves faster in acetone than ethanol, it is readily available, and it evaporates cleanly. Lab-grade (not hardware-store) acetone is preferred as impurities can cause yellowing. Lab-grade ethanol also works but can cause cloudiness at high humidity. Toluene gives different working properties and is more hazardous — only for specialist contexts. Always consult the Safety Data Sheet for your chosen solvent and work in a well-ventilated space.
How long does it take Paraloid B-72 pellets to dissolve?
In acetone, pellets can take several hours to fully dissolve even with stirring, because they tend to clump and settle. The cheesecloth bundle method — wrapping pellets in cheesecloth and suspending them in the solvent so they dissolve from the outside in — is recommended by Davidson & Brown (2012) and significantly speeds the process. A magnetic stirrer or regular manual stirring also helps. Ethanol takes longer still.
Why does my Paraloid solution keep getting thicker?
Solvent evaporates from any open container, increasing the effective concentration over time. Store solutions in airtight jars. If thickening has occurred, use the Evaporation Top-up tab to calculate how much fresh solvent to add to restore your target concentration. Adding solvent directly and stirring gently is sufficient — no need to start a new batch.
Can I use hardware-store acetone?
It is preferable to use laboratory-grade solvents. Hardware-store acetone commonly contains impurities or additives (such as denatonium in some denatured products) that may cause yellowing of B-72 over time or react with fossil minerals. For specimens of scientific or display value, source lab-grade acetone from a chemical supplier.