We Built a Volcano—and It Was Explosively Fun! The Ultimate Experiment Recipe Using Plaster and Baking Soda!
Hi, I’m Ken Kuwako, Science Trainer. Every day is an experiment.
Let’s be honest: the best part of science class is actually “trying it out.” Seeing students’ eyes light up and hearing a big “I get it!” makes us teachers just as happy. I taught a unit on volcanoes together with Mr. H, a student teacher at the time, and the eruption experiment we did turned out to be a real spark for curiosity.
“How does the stickiness of magma affect the shape of a volcano?”
The fun of science isn’t just memorizing what’s in the textbook. It’s about seeing, touching, and feeling things for yourself. So we decided to build our own volcanic eruption models from everyday materials! This experiment looks simple at first glance, but it’s surprisingly deep.
You only need four materials: plaster, baking soda, laundry starch glue, and water. Just by tweaking the mixing ratio of these four ingredients, the way the magma moves and how sticky it is changes completely. It feels like running a simulation of real volcanic activity.
What I find especially fascinating is the chemical reaction that happens when you combine plaster (calcium sulfate: CaSO₄), baking soda (sodium bicarbonate: NaHCO₃), and water. It produces carbon dioxide (CO₂)!
As the CO₂ forms, the mixture puffs up and swells, just like a real volcano slowly bulging before an eruption.
We tweaked the recipe again and again before we finally hit on one that made us say, “Oh, this might actually work!” It took a lot of time, honestly, but it reminded me that this is exactly what scientific inquiry is all about. And it didn’t stop there: the student teachers the following year, Mr. T and Mr. S, improved it even further. In this article, I’ll walk you through the ingredient ratios we arrived at after all that trial and error, and show you how a successful eruption looks!
What You’ll Need
・Plastic container ・Plaster 10 g × 2 ← put in a zip-top storage bag ・Baking soda 5 g × 2 ← put in an evaporating dish ・Laundry starch glue (PVA) ・Water ・Small spoon (medicine spoon) ・Glass stirring rod ・Tray ・Corrugated plastic sheet
For the plastic container, we used this one sold at Daiso. Size matters here, so if you want to follow the same method, please get the same one.
Daiso 70 ml, pack of 4, round mini containers
Use a soldering iron to poke a hole in the bottom of the plastic container and in the corrugated plastic sheet, then cover the hole in the container with cloth tape ahead of time.
It’s a good idea to keep everything in a shallow tray.
The size of the hole is the key. It should be about this big.
Now for the all-important recipe:
Low-viscosity magma (runny type): 1 ml PVA laundry starch glue + 30 ml water
High-viscosity magma (sticky type): 10 ml PVA laundry starch glue + 15 ml water
Prepare each mixture in advance in the containers above.
【Procedure】
① Stir the PVA glue and water in the plastic container well with the glass rod.
② Add 10 g of plaster (from the zip-top bag) to the container of PVA glue and water.
③ Stir with the glass rod until the plaster, PVA glue, and water are thoroughly mixed.
④ Once it’s well mixed, add 5 g of baking soda and stir quickly so everything blends together.
※ Don’t mix up the order of ③ and ④. Adding the baking soda in ④ is what triggers the chemical reaction that produces carbon dioxide. Also, if you leave the mixture at step ③ for 20 minutes or so, the plaster reacts with the water and hardens, and the experiment will fail. From what I found, plaster and water react and start to thicken and set in about 5 minutes.
※ In ④, it’s also important to mix everything quickly and put the lid on right away. Baking soda can sometimes clump together, so crush it finely with the small spoon beforehand.
⑤ Close the lid, flip the container upside down onto the tray, and immediately peel off the tape.
⑥ Cover it with the corrugated plastic sheet that has the hole, and look down at the lava model from directly above to watch how the liquid (lava) spreads.
※ The experimenter must wear polyethylene gloves! Everyone should also wear a lab coat and safety goggles!
Here’s how it actually turned out:
Low viscosity (runny)
High viscosity (sticky)
Top view, low viscosity
Top view, high viscosity
Here’s what happens without the corrugated plastic sheet. You can really see how much better it works with the sheet.
To sum up: the low-viscosity type is on the left, and the high-viscosity type is on the right.
And here’s a side-by-side comparison from above.
・Footage of an eruption with low-viscosity magma (Kilauea lava)
・An eruption with medium-viscosity magma (Sakurajima eruption)
Why Are the Colors Different?
Basalt from Mauna Loa and Kilauea vs. rhyolite from Showa-Shinzan (notice the difference in color)
Basalt is dark, while rhyolite is light-colored.
What Happens Without the Bottom Partition
High viscosity (sticky)
From above:
Low viscosity (runny)
Thanks to a clever idea from Mr. H, we placed both on a black tray and ran them side by side.
The runny type is at the top left, and the sticky type is on the right. We had groups of four share one set, and the room went wild.
【Why Does This Recipe Erupt?】
The key to this experiment is the chemical reaction between plaster, baking soda, and water.
CO₂ from the breakdown of baking soda: When baking soda (sodium bicarbonate: NaHCO₃) dissolves in water, part of it breaks down into carbonate ions (CO₃²⁻) and bicarbonate ions (HCO₃⁻), which then produce carbon dioxide (CO₂) and water.
CO₂ from the reaction between plaster and baking soda:
Calcium ions (Ca²⁺) released when plaster (calcium sulfate: CaSO₄) dissolves in water react with the bicarbonate ions (HCO₃⁻) from the baking soda. Calcium carbonate (CaCO₃) precipitates out, while carbon dioxide (CO₂) and water are produced at the same time.
The role of laundry starch glue (PVA): The polyvinyl alcohol (PVA) in laundry starch glue forms long, tangled molecules when dissolved in water, making the liquid thick and sticky. This PVA traps the CO₂ bubbles so they don’t escape all at once. Instead, the mixture swells slowly and oozes out, recreating a “sticky, viscous eruption.”
With only a little PVA, the bubbles escape quickly and you get a runny eruption (like the shield volcanoes of Hawaii). With a lot of PVA, the mixture gets very sticky and builds up into a dome shape (like the lava dome of Showa-Shinzan).
Contact and Requests
Bring the wonder and fun of science closer to everyday life! Here you’ll find fun science experiments you can do at home, along with tips to make them work, all explained in plain language. Try searching around the site!
・For more about the author, Ken Kuwako, click here.
・For requests (writing, lectures, science workshops, TV supervision or appearances, etc.), click here.
・Blog updates are posted on X!
The Science Gags Channel has experiment videos. Come check it out!
テレビ番組監修・イベント等のお知らせ
- 「ネイチャーティーチャー」（TBS）に出演しました。Tverにて11月3日（火）まで配信中です。
- サタプラ（MBS）にて「日常に潜む！ピンチ脱出法について」のコーナーに出演しました。Tverにて11月7日（土）まで配信中です。
- 11/4（水）ZIP（日本テレビ）のコーナー（7:20分ころ放送予定）にて科学監修を担当しました。ゴンチャのストローの差し方に関するウラザワと科学解説についてです。
- 11月27日（金） テレビ監修番組放送予定
- 12月26日（土） ナリカサイエンスアカデミー（教員向け実験講習会）開催
１１月のイチオシ実験！
どうして結露は起こるのか？結露を防ぐためには？実験で確かめよう！
書籍のお知らせ
- 『高校入試 分解問題集 理科』（学研）…難しい問題も小さな問題に分解することで、問題を解くことができます。そんな分解の技術が身につくように深く関わりを持って作りました。
『大人のための高校物理復習帳』（講談社）…一般向けに日常の物理について公式を元に紐解きました。特設サイトでは実験を多数紹介しています。※増刷がかかり６刷となりました（2026/02/01）
- 『きめる!共通テスト 物理基礎 改訂版』（学研）… 高校物理の参考書です。イラストを多くしてイメージが持てるように描きました。授業についていけない、物理が苦手、そんな生徒におすすめです。特設サイトはこちら。
各種SNS（更新情報をお届け！）
【日本語】X(Twitter)／instagram／Facebook 【英語】BlueSky／Threads
Explore
- 楽しい実験…お子さんと一緒に夢中になれるイチオシの科学実験を多数紹介しています。また、高校物理の理解を深めるための動画教材も用意しました。
- 理科の教材… 理科教師をバックアップ！授業の質を高め、準備を効率化するための選りすぐりの教材を紹介しています。
- Youtube…科学実験等の動画を配信しています。
- 科学ラジオ …科学トピックをほぼ毎日配信中！AI技術を駆使して作成した「耳で楽しむ科学」をお届けします。
- 講演 …全国各地で実験講習会・サイエンスショー等を行っています。
- About …「科学のネタ帳」のコンセプトや、運営者である桑子研のプロフィール・想いをまとめています。
- お問い合わせ …実験教室のご依頼、執筆・講演の相談、科学監修等はこちらのフォームからお寄せください。
