In the Java Edition lava-cast method, you build a temporary frame, place lava where the portal blocks belong, then use water to turn the lava into obsidian. That lets you create the portal without mining ten obsidian blocks with a diamond pickaxe. The cited tutorial presents this as a Java Edition speedrunning technique; it does not establish that the same sequence works across Bedrock Edition or every Java version.
What you need for the lava-cast method
- An iron bucket
- A lava source
- A water source
- Dirt or cobblestone for a temporary mold
- Flint and steel or a fire charge to ignite the finished frame
The method and dimensions below come from Astroworld’s community tutorial, “Fast Nether Portal Construction – Speedrun Tutorial,” which explicitly covers Java Edition speedrunning. Its version coverage is not stated.
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How to build the portal with lava and water
- Prepare the ignition item. Have flint and steel or a fire charge ready before you make the final cast, so you can light the portal as soon as the obsidian frame is complete.
- Make a temporary mold. Build a 4-block-wide by 5-block-tall outline from dirt or cobblestone, leaving an opening 2 blocks wide and 3 blocks tall. The corner blocks are optional; the dimensions refer to the frame’s overall width and height.
- Place lava in the frame positions. Use your bucket to put lava where each obsidian block in the frame should be. Follow the tutorial’s placement sequence carefully so the lava stays in the intended positions.
- Pour water over the lava. The lava should convert to obsidian where the portal frame blocks belong. Recover the water between placements as needed, rather than letting it interfere with later casts.
- Remove the temporary mold. Once the frame is made of obsidian, break away the dirt or cobblestone used to shape it.
- Ignite the frame. Use flint and steel or a fire charge on the inside of the completed frame to activate the portal.
What is the minimum Nether portal size?
The cited Java Edition tutorial gives a minimum frame size of 4 blocks wide by 5 blocks tall overall, with a 2-by-3-block opening. Corner blocks are optional, so you do not need to cast them. The guide’s dimensions describe the portal frame, not a requirement to build a solid 4-by-5 rectangle.
Why does lava turn to cobblestone instead of obsidian?
For this method, the tutorial’s expected result depends on casting the intended lava source blocks in the frame positions. If water reaches lava in the wrong way, or the placement sequence is off, the lava may not become the obsidian block you need. The guide also calls out two sources of wasted time: making the frame larger than necessary and forgetting to retrieve water between placements. These are the tutorial’s coaching points, not measured failure rates.
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Should you use a ruined portal instead?
A ruined portal can save construction work if its frame is close to usable and you can obtain enough additional obsidian. Minecraft’s official explanation says ruined-portal frames are incomplete and need added obsidian; nearby chests sometimes contain obsidian and gold items, not invariably obsidian. Inspect the frame and chest contents before choosing this route.
| What to check | Lava casting | Repairing a ruined portal |
|---|---|---|
| Materials and setup | Requires an iron bucket, lava, water, temporary blocks, and an ignition item, according to Astroworld’s Java Edition tutorial. | Requires enough additional obsidian to complete the generated frame; nearby chests sometimes contain obsidian, according to Minecraft.net’s “Building Blocks: Ruined Portal” (2024-12-04). |
| Work before activation | Cast the obsidian frame in place, remove the mold, then ignite it. | Inspect how much of the generated frame is missing, add obsidian, then ignite it. |
| Key uncertainty | The cited sequence is presented for Java Edition; applicability to Bedrock Edition and every Java version is not established. | The frame’s condition and chest contents vary; the official source does not say every chest contains obsidian or that loot always suffices. |
| Which is quicker? | No controlled timing comparison across seeds, editions, versions, or player skill is available in the cited sources. Choose based on the resources at hand and which setup you can execute consistently. | |
How the portal fits into a speedrun
Portal construction is one step in a larger route, not a guarantee of a particular completion time. Minecraft’s official under-30-minute speedrun article describes obtaining obsidian before entering the Nether, then continuing with objectives such as finding a fortress and building a blaze farm. It is broader route context, not evidence that one portal-building method is universally fastest.
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- Minecraft.net: “Building Blocks: Ruined Portal”, by Duncan Geere, published 2024-12-04.
- Minecraft.net: “How to Beat Minecraft in Under 30 Mins”, by Kate Gray, published 2022-05-18.
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